Remove NS special handling of 'frame-title-format'

Remove NS special handling of 'frame-title-format'

* lisp/term/ns-win.el (frame-title-format, icon-title-format): Change
default format to just the filename.
* src/nsfns.m (ns-use-proxy-icon): New variable.
(ns_set_name_as_filename): Remove function.
(x_implicitly_set_name): Get rid of special handling of
frame-title-format.
* src/nsterm.h (ns_set_represented_filename): Use new definition.
* src/nsterm.m (ns_set_represented_filename): Move some of the logic
from ns_set_name_as_filename into this function.