libstdc++: Remove redundant -std=gnu++17 option from PMR 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/20_util/memory_resource/1.cc: Remove -std=gnu++17
	from dg-options directive.
	* testsuite/20_util/memory_resource/2.cc: Likewise.
	* testsuite/20_util/monotonic_buffer_resource/1.cc: Likewise.
	* testsuite/20_util/monotonic_buffer_resource/93208.cc:
	Likewise.
	* testsuite/20_util/monotonic_buffer_resource/allocate.cc:
	Likewise.
	* testsuite/20_util/monotonic_buffer_resource/deallocate.cc:
	Likewise.
	* testsuite/20_util/monotonic_buffer_resource/release.cc:
	Likewise.
	* testsuite/20_util/monotonic_buffer_resource/upstream_resource.cc:
	Likewise.
	* testsuite/20_util/polymorphic_allocator/1.cc: Likewise.
	* testsuite/20_util/polymorphic_allocator/construct_pair.cc:
	Likewise.
	* testsuite/20_util/polymorphic_allocator/resource.cc: Likewise.
	* testsuite/20_util/polymorphic_allocator/select.cc: Likewise.
	* testsuite/20_util/synchronized_pool_resource/allocate.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/allocate_single.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/cons.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/cons_single.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/is_equal.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/multithreaded.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/options.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/release.cc:
	Likewise.
	* testsuite/20_util/synchronized_pool_resource/release_single.cc:
	Likewise.
	* testsuite/20_util/unsynchronized_pool_resource/allocate-max-chunks.cc:
	Likewise.
	* testsuite/20_util/unsynchronized_pool_resource/allocate.cc:
	Likewise.
	* testsuite/20_util/unsynchronized_pool_resource/cons.cc:
	Likewise.
	* testsuite/20_util/unsynchronized_pool_resource/is_equal.cc:
	Likewise.
	* testsuite/20_util/unsynchronized_pool_resource/options.cc:
	Likewise.
	* testsuite/20_util/unsynchronized_pool_resource/release.cc:
	Likewise.
	* testsuite/21_strings/basic_string/types/pmr_typedefs.cc:
	Likewise.
	* testsuite/23_containers/deque/types/pmr_typedefs.cc: Likewise.
	* testsuite/23_containers/deque/types/pmr_typedefs_debug.cc:
	Likewise.
	* testsuite/23_containers/forward_list/pmr_typedefs.cc:
	Likewise.
	* testsuite/23_containers/forward_list/pmr_typedefs_debug.cc:
	Likewise.
	* testsuite/23_containers/list/pmr_typedefs.cc: Likewise.
	* testsuite/23_containers/list/pmr_typedefs_debug.cc: Likewise.
	* testsuite/23_containers/map/pmr_typedefs.cc: Likewise.
	* testsuite/23_containers/map/pmr_typedefs_debug.cc: Likewise.
	* testsuite/23_containers/multimap/pmr_typedefs.cc: Likewise.
	* testsuite/23_containers/multimap/pmr_typedefs_debug.cc:
	Likewise.
	* testsuite/23_containers/multiset/pmr_typedefs.cc: Likewise.
	* testsuite/23_containers/multiset/pmr_typedefs_debug.cc:
	Likewise.
	* testsuite/23_containers/set/pmr_typedefs.cc: Likewise.
	* testsuite/23_containers/set/pmr_typedefs_debug.cc: Likewise.
	* testsuite/23_containers/unordered_map/pmr_typedefs.cc:
	Likewise.
	* testsuite/23_containers/unordered_map/pmr_typedefs_debug.cc:
	Likewise.
	* testsuite/23_containers/unordered_multimap/pmr_typedefs.cc:
	Likewise.
	* testsuite/23_containers/unordered_multimap/pmr_typedefs_debug.cc:
	Likewise.
	* testsuite/23_containers/unordered_multiset/pmr_typedefs.cc:
	Likewise.
	* testsuite/23_containers/unordered_multiset/pmr_typedefs_debug.cc:
	Likewise.
	* testsuite/23_containers/unordered_set/pmr_typedefs.cc:
	Likewise.
	* testsuite/23_containers/unordered_set/pmr_typedefs_debug.cc:
	Likewise.
	* testsuite/23_containers/vector/pmr_typedefs.cc: Likewise.
	* testsuite/23_containers/vector/types/pmr_typedefs_debug.cc:
	Likewise.
	* testsuite/28_regex/match_results/pmr_typedefs.cc: Likewise.