Omit unused frame argument of font API's close function.

Omit unused frame argument of font API's close function.
* font.h (struct font): Drop frame argument.  Adjust comment.
* font.c (font_clear_cache, font_close_object): Adjust users.
* ftfont.c (ftfont_close):
* ftxfont.c (ftxfont_close):
* macfont.m (macfont_close):
* nsfont.m (nsfont_close):
* w32font.c (w32font_close):
* xfont.c (xfont_close):
* xftfont.c (xftfont_close): Adjust driver-specific close
functions, tweak comments and make functions safe if called
more than once for the same font object.