libstdc++: Remove redundant -std=gnu++17 option from algorithm 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/function_objects/searchers.cc: Remove
	-std=gnu++17 from dg-options directive.
	* testsuite/20_util/specialized_algorithms/memory_management_tools/1.cc:
	Likewise.
	* testsuite/20_util/specialized_algorithms/memory_management_tools/destroy_neg.cc:
	Likewise.
	* testsuite/20_util/specialized_algorithms/uninitialized_value_construct/94540.cc:
	Likewise.
	* testsuite/20_util/specialized_algorithms/uninitialized_value_construct/94831.cc:
	Likewise.
	* testsuite/20_util/specialized_algorithms/uninitialized_value_construct_n/94540.cc:
	Likewise.
	* testsuite/20_util/specialized_algorithms/uninitialized_value_construct_n/sizes.cc:
	Likewise.
	* testsuite/20_util/unique_ptr/specialized_algorithms/swap_cxx17.cc:
	Likewise.
	* testsuite/25_algorithms/clamp/1.cc: Likewise.
	* testsuite/25_algorithms/clamp/2.cc: Likewise.
	* testsuite/25_algorithms/clamp/constexpr.cc: Likewise.
	* testsuite/25_algorithms/clamp/requirements/explicit_instantiation/1.cc:
	Likewise.
	* testsuite/25_algorithms/clamp/requirements/explicit_instantiation/pod.cc:
	Likewise.
	* testsuite/25_algorithms/for_each/for_each_n.cc: Likewise.
	* testsuite/25_algorithms/for_each/for_each_n_debug.cc:
	Likewise.
	* testsuite/25_algorithms/sample/1.cc: Likewise.
	* testsuite/25_algorithms/sample/2.cc: Likewise.
	* testsuite/25_algorithms/sample/3.cc: Likewise.
	* testsuite/25_algorithms/sample/81221.cc: Likewise.
	* testsuite/25_algorithms/search/searcher.cc: Likewise.
	* testsuite/26_numerics/exclusive_scan/1.cc: Likewise.
	* testsuite/26_numerics/inclusive_scan/1.cc: Likewise.
	* testsuite/26_numerics/reduce/1.cc: Likewise.
	* testsuite/26_numerics/reduce/2.cc: Likewise.
	* testsuite/26_numerics/transform_exclusive_scan/1.cc: Likewise.
	* testsuite/26_numerics/transform_inclusive_scan/1.cc: Likewise.
	* testsuite/26_numerics/transform_reduce/1.cc: Likewise.