doc: posix: remove unneeded notes about XSI_THREADS_EXT

Previously, XSI_THREADS_EXT included a note about
undefined behaviour. However, the required functions
pthread_attr_getstack(), pthread_attr_setstack(),
pthread_getconcurrency(), and pthread_setconcurrency()
are all conformant.

Remove the unneeded note (all of them)

Signed-off-by: Chris Friedt <chrisfriedt@gmail.com>