Simplify Objective C autorelease pool handling

Simplify Objective C autorelease pool handling

* src/emacs.c: Remove ns_pool.
(main): Replace ns_pool stuff with call to ns_init_pool.
(Fkill_emacs): The pools are drained automatically when the
application exits, so it's probably not worth draining the pool here.
(decode_env_path): No longer required as this is handled by
`outerpool' defined in nsterm.m.
* src/nsterm.h:
* src/nsterm.m (ns_init_pool): New function.