libstdc++: Remove redundant -std=gnu++17 option from concurrency tests

libstdc++: Remove redundant -std=gnu++17 option from concurrency tests

GCC defaults to -std=gnu++17 now anyway, and using it explicitly in the
dg-options directive prevents running these tests with different modes
such as -std=c++17 or -std=gnu++20.

libstdc++-v3/ChangeLog:

	* testsuite/29_atomics/atomic/69769.cc: Remove -std=gnu++17 from
	dg-options.
	* testsuite/29_atomics/atomic/is_always_lock_free.cc:
	* testsuite/29_atomics/atomic/requirements/typedefs.cc:
	* testsuite/29_atomics/atomic_integral/is_always_lock_free.cc:
	* testsuite/29_atomics/atomic_integral/requirements/typedefs.cc:
	* testsuite/30_threads/lock_guard/cons/deduction.cc: Likewise.
	* testsuite/30_threads/scoped_lock/cons/1.cc: Likewise.
	* testsuite/30_threads/scoped_lock/cons/deduction.cc: Likewise.
	* testsuite/30_threads/scoped_lock/requirements/explicit_instantiation.cc:
	Likewise.
	* testsuite/30_threads/scoped_lock/requirements/typedefs.cc:
	Likewise.
	* testsuite/30_threads/shared_lock/70766.cc: Likewise.
	* testsuite/30_threads/shared_mutex/cons/1.cc: Likewise.
	* testsuite/30_threads/shared_mutex/cons/assign_neg.cc:
	Likewise.
	* testsuite/30_threads/shared_mutex/cons/copy_neg.cc: Likewise.
	* testsuite/30_threads/shared_mutex/requirements/standard_layout.cc:
	Likewise.
	* testsuite/30_threads/shared_mutex/try_lock/1.cc: Likewise.
	* testsuite/30_threads/shared_mutex/try_lock/2.cc: Likewise.
	* testsuite/30_threads/shared_mutex/unlock/1.cc: Likewise.
	* testsuite/30_threads/unique_lock/cons/deduction.cc: Likewise.