libstdc++/regex: Remove now unused __dfs_mode template parameter

libstdc++/regex: Remove now unused __dfs_mode template parameter

This mechanical patch removes _Executor's __dfs_mode template parameter.
In passing we can also get rid of the _V2 inline namespace because this
removal alone achieve the goal of the inline namespace which is to make
all of _Executor's member function signatures different from the
previous (GCC < 16) recursive implementation, allowing for safe mixing
of the two incompatible implementations.

libstdc++-v3/ChangeLog:

	* include/bits/regex.h (_Executor): Remove __dfs_mode template
	parameter and _V2 inline namespace.
	* include/bits/regex.tcc (__regex_algo_impl): Adjust after
	__dfs_mode template parameter removal.
	* include/bits/regex_executor.h (_Executor): Remove __dfs_mode
	parameter and _V2 inline namespace.
	* include/bits/regex_executor.tcc (_Executor): Likewise.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>