crt: Remove dummy __C_specific_handler implementation

mingw-w64 crt code does not use __C_specific_handler function since the
commit 271ce1baac1df4a062a419cd57120b4897c2fdfb ("crt: crt_handler: Switch
from __C_specific_handler+_gnu_exception_handler to __mingw_SEH_error_handler").

The current __C_specific_handler implementation is incomplete and therefore
it is not a good idea to pretend it is useful for applications.
So remove this incomplete / dummy implementation of __C_specific_handler.

Signed-off-by: LIU Hao <lh_mouse@126.com>