libstdc++: Remove dg-options "-std=gnu++20" from 30_threads 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/30_threads/barrier/1.cc: Remove dg-options
	-std=gnu++20.
	* testsuite/30_threads/barrier/2.cc: Likewise.
	* testsuite/30_threads/barrier/arrive.cc: Likewise.
	* testsuite/30_threads/barrier/arrive_and_drop.cc: Likewise.
	* testsuite/30_threads/barrier/arrive_and_wait.cc: Likewise.
	* testsuite/30_threads/barrier/completion.cc: Likewise.
	* testsuite/30_threads/condition_variable/members/clock_neg.cc:
	Likewise.
	* testsuite/30_threads/condition_variable_any/members/clock_neg.cc:
	Likewise.
	* testsuite/30_threads/condition_variable_any/stop_token/1.cc:
	Likewise.
	* testsuite/30_threads/condition_variable_any/stop_token/2.cc:
	Likewise.
	* testsuite/30_threads/future/members/clock_neg.cc: Likewise.
	* testsuite/30_threads/headers/stop_token/synopsis.cc: Likewise.
	* testsuite/30_threads/headers/thread/types_std_c++20.cc:
	Likewise.
	* testsuite/30_threads/jthread/1.cc: Likewise.
	* testsuite/30_threads/jthread/100612.cc: Likewise.
	* testsuite/30_threads/jthread/95989.cc: Likewise.
	* testsuite/30_threads/jthread/jthread.cc: Likewise.
	* testsuite/30_threads/jthread/version.cc: Likewise.
	* testsuite/30_threads/latch/1.cc: Likewise.
	* testsuite/30_threads/latch/2.cc: Likewise.
	* testsuite/30_threads/latch/3.cc: Likewise.
	* testsuite/30_threads/recursive_timed_mutex/try_lock_until/clock_neg.cc:
	Likewise.
	* testsuite/30_threads/semaphore/1.cc: Likewise.
	* testsuite/30_threads/semaphore/2.cc: Likewise.
	* testsuite/30_threads/semaphore/least_max_value_neg.cc:
	Likewise.
	* testsuite/30_threads/semaphore/try_acquire_until.cc: Likewise.
	* testsuite/30_threads/shared_future/members/clock_neg.cc:
	Likewise.
	* testsuite/30_threads/shared_lock/locking/clock_neg.cc:
	Likewise.
	* testsuite/30_threads/shared_timed_mutex/try_lock_until/clock_neg.cc:
	Likewise.
	* testsuite/30_threads/stop_token/1.cc: Likewise.
	* testsuite/30_threads/stop_token/2.cc: Likewise.
	* testsuite/30_threads/stop_token/stop_callback.cc: Likewise.
	* testsuite/30_threads/stop_token/stop_callback/deadlock.cc:
	Likewise.
	* testsuite/30_threads/stop_token/stop_callback/destructible_neg.cc:
	Likewise.
	* testsuite/30_threads/stop_token/stop_callback/invocable_neg.cc:
	Likewise.
	* testsuite/30_threads/stop_token/stop_callback/invoke.cc:
	Likewise.
	* testsuite/30_threads/stop_token/stop_source.cc: Likewise.
	* testsuite/30_threads/stop_token/stop_source/assign.cc:
	Likewise.
	* testsuite/30_threads/stop_token/stop_token.cc: Likewise.
	* testsuite/30_threads/stop_token/stop_token/stop_possible.cc:
	Likewise.
	* testsuite/30_threads/thread/id/operators_c++20.cc: Likewise.
	* testsuite/30_threads/timed_mutex/try_lock_until/clock_neg.cc:
	Likewise.
	* testsuite/30_threads/unique_lock/locking/clock_neg.cc:
	Likewise.