libstdc++: Remove redundant overflow check for futex timeout [PR 93456]

libstdc++: Remove redundant overflow check for futex timeout [PR 93456]

The relative_timespec function already checks for the case where the
specified timeout is in the past, so the difference can never be
negative. That means we dn't need to check if it's more negative than
the minimum time_t value.

libstdc++-v3/ChangeLog:

	PR libstdc++/93456
	* src/c++11/futex.cc (relative_timespec): Remove redundant check
	negative values.
	* testsuite/30_threads/future/members/wait_until_overflow.cc: Moved to...
	* testsuite/30_threads/future/members/93456.cc: ...here.