libstdc++: Remove unnecessary string in filesystem::path formatter

libstdc++-v3/ChangeLog:

	* include/bits/fs_path.h (std::formatter<filesystem::path, _CharT>):
	Format _Utf_view directly via __formatter_str::_M_format_range.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>