make_cleanup_py_decref and make_cleanup_py_xdecref are now unused, so
this patch removes themm.  Future Python changes should use gdbpy_ref
instead.

2017-01-10  Tom Tromey  <tom@tromey.com>

	* python/python-internal.h (make_cleanup_py_decref)
	(make_cleanup_py_xdecref): Don't declare.
	* python/py-utils.c (py_decref, make_cleanup_py_decref)
	(py_xdecref, make_cleanup_py_xdecref): Remove.