Further cleanup of NS color code

Further cleanup of NS color code

* src/dispextern.h (FACE_COLOR_TO_PIXEL): Remove define and fix all
callers.
* src/nsterm.h (struct nsfont_info): Remove color_table.
* src/nsterm.m ([NSColor colorWithUnsignedLong:]): Always assume the
input contains the correct alpha value.
(ns_lookup_indexed_color):
(ns_index_color):
(ns_color_index_to_rgba): Remove functions and fix all callers.
(ns_query_color): No longer set pixel to the lookup table index,
always just set it to the ARGB integer value.
(ns_defined_color): Ignore makeindex as we no longer set pixel to the
lookup table index.
(ns_initialize_display_info):
(ns_redisplay_interface): Remove lookup table.
(ns_term_init): Fix the alpha setting.

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