libc: Do not refer to _DefaultRuneLocale in ctype inlines

Referring to _DefaultRuneLocale causes this >4KB structure to be copied to
all executables that use <ctype.h> inlines (except PIE executables).

This only affects the case where thread local storage is available.

_CurrentRuneLocale cannot be NULL, so the check can be removed entirely.

_DefaultRuneLocale needs to remain available for now since libc++ uses it.
The __isctype inline in include/_ctype.h also refers to _DefaultRuneLocale
and remains available because it may still be used by third party software.

Reviewed by: bdrewery, theraven
Differential Revision: https://reviews.freebsd.org/D10363