winpthreads: remove WINPTHREAD_DBG macro

The `WINPTHREAD_DBG` macro guards internal functions which are
no longer used. Code guarded by this macro has been removed.

This commit also removed private definition of `assert` macro
from misc.h. assert.h has been included from affected source files.

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