libstdc++: Simplify some chrono formatters

I don't remember exactly why I made these bits of code reserve space in
a COW string and append to it, rather than just use the string returned
from std::format (which will undergo copy elision). The _Str_sink type
used by std::format means the string only performs a single allocation
for the formatted output, and the returned string's reference count will
be one, so won't reallocate when indexing into it. We can remove these
non-optimizations.

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__formatter_chrono::_M_F): Simplify
	handling of string returned from std::format.
	(__formatter_chrono::_M_R_T): Likewise.