Simplify and cleanup character conversion stuff.

Simplify and cleanup character conversion stuff.
* lisp.h (multibyte_char_to_unibyte, multibyte_char_to_unibyte_safe):
Remove prototypes.
* character.c (multibyte_char_to_unibyte)
(multibyte_char_to_unibyte_safe): Remove; no longer used.
* character.h (make_char): Remove; unused.
(CHAR_TO_BYTE8, CHAR_TO_BYTE_SAFE): Simplify.
(ASCII_BYTE_P): Remove; ASCII_CHAR_P does the same thing.
* buffer.c, charset.c, charset.h, cmds.c, coding.c, editfns.c:
* fileio.c, indent.c, insdel.c, keyboard.c, lread.c, print.c:
* search.c, term.c, xdisp.c, xterm.c: Related users changed.