libstdc++: Remove std::formatter<const charT[N], charT> specialization

This was approved in Issaquah as LWG 3833.

libstdc++-v3/ChangeLog:

	* include/std/format (formatter<const charT[N], charT>): Do not
	define partial speclialization, as per LWG 3833.
	* testsuite/std/format/formatter/requirements.cc: Check it.