Throughout the file, delete all USE_FONT_BACKEND

Throughout the file, delete all USE_FONT_BACKEND
conditionals.  Don't check enable_font_backend.  Delete all codes
used only when USE_FONT_BACKEND is not defined.  Don't include
ccl.h.
(x_per_char_metric, x_encode_char): Deleted.
(x_set_cursor_gc, x_set_mouse_face_gc): Don't set GCFont.
(x_compute_glyph_string_overhangs): Adjusted for the change of
`struct face'
(x_draw_glyph_string_foreground)
(x_draw_composite_glyph_string_foreground): Likewise.
(x_draw_glyph_string): Likewise.  Use font->underline_position and
font->underline_thickness.
(x_new_font): Renamed from x_new_fontset2.
(x_new_fontset, x_get_font_info, x_list_fonts): Deleted.
(x_check_font): Call `check' method of a font driver.
(x_font_min_bounds, x_compute_min_glyph_bounds, x_load_font)
(x_query_font, x_get_font_repertory): Deleted.
(x_find_ccl_program): Renamed and moved to xfont.c.
(x_redisplay_interface): Adjusted for the change of `struct
redisplay_interface'.