libstdc++: Remove duplicate test

libstdc++: Remove duplicate test

We currently have 808590.cc which only runs for C++98 mode, and
808590-cxx11.cc which only runs for C++11 and later, but have almost
identical content (except for a defaulted special member in the C++11
one, to suppress a -Wdeprecated-copy warning).

This was done originally to ensure that the test ran for both C++98 mode
and C++11 mode, because the logic being tested was different enough to
need both to be tested. But it's trivial to run all tests in multiple
-std modes now, using GLIBCXX_TESTSUITE_STDS, so we don't need two
separate tests. We can remove one of the tests and allow the other one
to run in any -std mode.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/specialized_algorithms/uninitialized_copy/808590.cc:
	Copy defaulted assignment operator from 808590-cxx11.cc to
	suppress a warning.
	* testsuite/20_util/specialized_algorithms/uninitialized_copy/808590-cxx11.cc:
	Removed.