Next minor cleanup of font subsystem.

Next minor cleanup of font subsystem.
* font.h (enum font_property_index): Remove FONT_ENTITY_INDEX (no
users) and FONT_FORMAT_INDEX (set by a few font drivers but never
really used).
(FONT_ENTITY_NOT_LOADABLE, FONT_ENTITY_SET_NOT_LOADABLE): Remove;
unused.
* ftfont.h (ftfont_font_format): Remove prototype.
* ftfont.c (ftfont_font_format): Remove; now unused.
(ftfont_open):
* nsfont.m (nsfont_open):
* w32font.c (w32font_open_internal):
* w32uniscribe.c (uniscribe_open):
* xfont.c (xfont_open):
* xftfont.c (xftfont_open): All users changed.