posix: remove inaccurate comment re initializers

A dangling `FIXME` comment was left in the `pthread.h` header
about how it would be difficult to implement
`PTHREAD_MUTEX_INITIALIZER` and `PTHREAD_COND_INITIALIZER`.

They were both implemented months ago, so let's retire
that comment.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>