Remove ensure_python_env

Remove ensure_python_env

All of gdb has been converted away from ensure_python_env and
varobj_ensure_python_env now; so remove them.

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

	* python/python.c (ensure_python_env, restore_python_env):
	Remove.
	* python/python-internal.h (ensure_python_env): Don't declare.
	* varobj.h (varobj_ensure_python_env): Don't declare.
	* varobj.c (varobj_ensure_python_env): Remove.