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.  Use
FONT_XXX_NAME_NUMERIC instead of face_numeric_xxx.
(QCfoundry, QCadstyle, QCregistry, QCspacing, QCsize, QCavgwidth)
(Qp): Extern them.
(clear_font_table, load_face_font, xlfd_lookup_field_contents):
Deleted.
(struct font_name): Deleted.
(xlfd_numeric_value, xlfd_symbolic_value): Deleted.
(compare_fonts_by_sort_order): New function.
(xlfd_numeric_slant, xlfd_symbolic_slant, xlfd_numeric_weight)
(xlfd_symbolic_weight, xlfd_numeric_swidth, xlfd_symbolic_swidth):
Deleted.
(Fx_family_fonts): Use font_list_entities, and sort fonts by
compare_fonts_by_sort_order.
(Fx_font_family_list): Call Ffont_family_list.
(face_numeric_value, face_numeric_weight, face_numeric_slant)
(face_numeric_swidth, face_symbolic_value, face_symbolic_weight)
(face_symbolic_slant, face_symbolic_swidth)
(split_font_name_into_vector, build_font_name_from_vector)
(xlfd_fixed_p, xlfd_point_size, pixel_point_size)
(font_rescale_ratio, split_font_name, build_font_name)
(free_font_names, sort_fonts, x_face_list_fonts)
(face_font_available_p, sorted_font_list, cmp_font_names)
(font_list_1, concat_font_list, font_list, remove_duplicates):
Deleted.
(Fx_list_fonts): Use Ffont_list.
(LFACE_AVGWIDTH): Deleted.
(check_lface_attrs): Don't check LFACE_AVGWIDTH.  Check LFACE_FONT
by FONTP.
(lface_fully_specified_p): Don't check LFACE_AVGWIDTH.
(set_lface_from_font_name): Delete it.
(set_lface_from_font): Renamed from
set_lface_from_font_and_fontset.  Caller changed.  Don't set
LFACE_AVGWIDTH.  Use FONT_XXX_FOR_FACE to get a symbol suitable
for face.
(merge_face_vectors): Copy font-spec if necessary.  Clear
properties of the font-spec if necessary.
(merge_face_ref): Clear properties of the font-spec if necessary.
(Finternal_set_lisp_face_attribute): Likewise.
(set_font_frame_param): Use font_load_for_lface to load a
font-object, and call Fmodify_frame_parameters with it.
(x_update_menu_appearance): Don't check LFACE_AVGWIDTH.  Get XLFD
font name by Ffont_xlfd_name.
(Finternal_lisp_face_attribute_values): Don't check QCweight,
QCslant, and QCwidth.
(Fface_font): Get a font name from font->props[FONT_NAME_INDEX].
(lface_same_font_attributes_p): Don't check LFACE_AVGWIDTH.
Compare fonts by EQ.
(lookup_non_ascii_face): Deleted.
(face_for_font): The 2nd argument changed.
(x_supports_face_attributes_p): Don't check LFACE_AVGWIDTH.  Check
atomic font properties by case insensitive.
(realize_non_ascii_face): Set face->overstrike correctly.
(realize_x_face): Likewise.  Check if LFACE_FONT is a font_object.
(dump_realized_face): Get font name from
font->props[FONT_NAME_INDEX].  Don't print font_info_id.