Remove outdated code from extensions.autoreload

imp.getsuffixes() raises a DeprecationWarning under Python 3.3;
in order to fix this I just removed _get_compiled_ext and
PY_COMPILED_EXT because they are no longer used
(see 8a8884b399cf3f5b761313bf0d446ba204f2c810 and 2911f3ed7db45ec2571772a9e25513559d480179).