libstdc++: Remove dg-options "-std=gnu++20" from std::chrono tests

libstdc++: Remove dg-options "-std=gnu++20" from std::chrono tests

The testsuite will automatically select C++20 for these tests now, and
removing the hardcoded -std option allows them to be tested for C++23
and C++26 as well.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/duration/arithmetic/overflow_c++20.cc:
	Remove dg-options.
	* testsuite/20_util/duration/io.cc: Likewise.
	* testsuite/std/time/clock/file/io.cc: Likewise.
	* testsuite/std/time/clock/file/members.cc: Likewise.
	* testsuite/std/time/clock/file/overview.cc: Likewise.
	* testsuite/std/time/clock/gps/1.cc: Likewise.
	* testsuite/std/time/clock/gps/io.cc: Likewise.
	* testsuite/std/time/clock/local/io.cc: Likewise.
	* testsuite/std/time/clock/system/io.cc: Likewise.
	* testsuite/std/time/clock/tai/1.cc: Likewise.
	* testsuite/std/time/clock/tai/io.cc: Likewise.
	* testsuite/std/time/clock/utc/1.cc: Likewise.
	* testsuite/std/time/clock/utc/io.cc: Likewise.
	* testsuite/std/time/clock/utc/leap_second_info.cc: Likewise.
	* testsuite/std/time/day/1.cc: Likewise.
	* testsuite/std/time/day/io.cc: Likewise.
	* testsuite/std/time/exceptions.cc: Likewise.
	* testsuite/std/time/format.cc: Likewise.
	* testsuite/std/time/hh_mm_ss/1.cc: Likewise.
	* testsuite/std/time/hh_mm_ss/109772.cc: Likewise.
	* testsuite/std/time/hh_mm_ss/io.cc: Likewise.
	* testsuite/std/time/is_am/1.cc: Likewise.
	* testsuite/std/time/is_pm/1.cc: Likewise.
	* testsuite/std/time/make12/1.cc: Likewise.
	* testsuite/std/time/make24/1.cc: Likewise.
	* testsuite/std/time/month/1.cc: Likewise.
	* testsuite/std/time/month/io.cc: Likewise.
	* testsuite/std/time/month_day/1.cc: Likewise.
	* testsuite/std/time/month_day/io.cc: Likewise.
	* testsuite/std/time/month_day_last/1.cc: Likewise.
	* testsuite/std/time/month_day_last/io.cc: Likewise.
	* testsuite/std/time/month_weekday/1.cc: Likewise.
	* testsuite/std/time/month_weekday/io.cc: Likewise.
	* testsuite/std/time/month_weekday_last/1.cc: Likewise.
	* testsuite/std/time/month_weekday_last/io.cc: Likewise.
	* testsuite/std/time/parse.cc: Likewise.
	* testsuite/std/time/syn_c++20.cc: Likewise.
	* testsuite/std/time/time_zone/get_info_local.cc: Likewise.
	* testsuite/std/time/time_zone/get_info_sys.cc: Likewise.
	* testsuite/std/time/time_zone/requirements.cc: Likewise.
	* testsuite/std/time/traits/is_clock.cc: Likewise.
	* testsuite/std/time/tzdb/1.cc: Likewise.
	* testsuite/std/time/tzdb/leap_seconds.cc: Likewise.
	* testsuite/std/time/tzdb_list/1.cc: Likewise.
	* testsuite/std/time/tzdb_list/requirements.cc: Likewise.
	* testsuite/std/time/weekday/1.cc: Likewise.
	* testsuite/std/time/weekday/io.cc: Likewise.
	* testsuite/std/time/weekday_indexed/io.cc: Likewise.
	* testsuite/std/time/weekday_last/1.cc: Likewise.
	* testsuite/std/time/weekday_last/io.cc: Likewise.
	* testsuite/std/time/year/1.cc: Likewise.
	* testsuite/std/time/year/2.cc: Likewise.
	* testsuite/std/time/year/io.cc: Likewise.
	* testsuite/std/time/weekday_indexed/1.cc: Likewise.
	* testsuite/std/time/year_month/1.cc: Likewise.
	* testsuite/std/time/year_month/2.cc: Likewise.
	* testsuite/std/time/year_month/io.cc: Likewise.
	* testsuite/std/time/year_month_day/1.cc: Likewise.
	* testsuite/std/time/year_month_day/2.cc: Likewise.
	* testsuite/std/time/year_month_day/3.cc: Likewise.
	* testsuite/std/time/year_month_day/4.cc: Likewise.
	* testsuite/std/time/year_month_day/io.cc: Likewise.
	* testsuite/std/time/year_month_day_last/1.cc: Likewise.
	* testsuite/std/time/year_month_day_last/2.cc: Likewise.
	* testsuite/std/time/year_month_day_last/io.cc: Likewise.
	* testsuite/std/time/year_month_weekday/1.cc: Likewise.
	* testsuite/std/time/year_month_weekday/2.cc: Likewise.
	* testsuite/std/time/year_month_weekday/3.cc: Likewise.
	* testsuite/std/time/year_month_weekday/io.cc: Likewise.
	* testsuite/std/time/year_month_weekday_last/1.cc: Likewise.
	* testsuite/std/time/year_month_weekday_last/2.cc: Likewise.
	* testsuite/std/time/year_month_weekday_last/io.cc: Likewise.
	* testsuite/std/time/zoned_time/1.cc: Likewise.
	* testsuite/std/time/zoned_time/custom.cc: Likewise.
	* testsuite/std/time/zoned_time/deduction.cc: Likewise.
	* testsuite/std/time/zoned_time/io.cc: Likewise.
	* testsuite/std/time/zoned_time/req_neg.cc: Likewise.
	* testsuite/std/time/zoned_time/requirements.cc: Likewise.
	* testsuite/std/time/zoned_traits.cc: Likewise.