winpthreads: remove some internal functions from pthread.h

Declarations of the following functions have been removed from pthread.h:

  - _pthread_rel_time_in_ms
  - _pthread_time_in_ms
  - _pthread_time_in_ms_from_timespec

These functions are declared in internal misc.h.

Signed-off-by: Kirill Makurin <maiddaisuki@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>