Minor cleanup of fonset code.

Minor cleanup of fonset code.
* fontset.c (FONTSET_ID, set_fontset_id, FONTSET_NAME)
(set_fontset_name, FONTSET_ASCII, set_fontset_ascii)
(FONTSET_BASE, set_fontset_base, FONTSET_FRAME)
(set_fontset_frame, FONTSET_NOFONT_FACE, set_fontset_nofont_face)
(FONTSET_DEFAULT, set_fontset_default, FONTSET_FALLBACK)
(set_fontset_fallback): Reorder extra slots and avoid unused slots.
(free_realized_fontset): Remove because a no-op since 2008.
(free_face_fontset): Adjust user.
(syms_of_fontset): Shrink fontset by one extra slot.