libc/common: Remove redundant Kconfig POSIX_THREAD_SAFE_FUNCTIONS uses

POSIX_THREAD_SAFE_FUNCTIONS already selects POSIX_C_LANG_SUPPORT_R and
POSIX_C_LANG_SUPPORT_R alread selects all of the COMMON_LIBC_*_R values.
That makes the 'default y if POSIX_THREAD_SAFE_FUNCTIONS' clauses in these
symbols redundant.

Signed-off-by: Keith Packard <keithp@keithp.com>