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The ns font backend is has been disabled on macOS for a long time and
doesn't work correctly even if re-enabled.

* src/nsfont.m:
(ns_char_width):
(ns_ascii_average_width):
(ns_get_covering_families):
(nsfont_open):
(nsfont_close):
(nsfont_draw):
(ns_uni_to_glyphs):
(ns_glyph_metrics):
(EmacsGlyphStorage): Remove all Cocoa only code.
* src/nsterm.h (EmacsGlyphStorage): Remove.
(struct nsfont_info): Make GNUstep only.
* src/nsterm.m (ns_compute_glyph_string_overhangs): Remove GNUstep
only code from Cocoa builds.