libstdc++: Remove unnecessary 'static' from __is_specialization_of

This makes the declarations internal linkage, which is an ODR issue, and
causes a future modules patch to fail regtest as it now detects attempted
uses of TU-local entities in module CMIs.

libstdc++-v3/ChangeLog:

	* include/std/format: Remove unnecessary 'static'.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>