cogl: remove OSX/WIN32 specific bits in favour of a stub winsys

cogl: remove OSX/WIN32 specific bits in favour of a stub winsys

Until Cogl gains native win32/OSX support this remove the osx and win32
winsys files and instead we'll just rely on the stub-winsys.c to handle
these platforms. Since the only thing the platform specific files were
providing anyway was a get_proc_address function; it was trivial to
simply update the clutter backend code to handle this directly for now.