Simplify str_to_multibyte and related code

* src/character.h (str_to_multibyte):
* src/character.c (str_to_multibyte): Remove `nbytes` argument; return
it instead.  Copy forwards.
* src/fns.c (concat_to_string, Fstring_make_multibyte):
Use str_to_multibyte.
(string_make_multibyte): Remove.
(string_to_multibyte):
* src/print.c (print_string):  Adapt calls.

lisp/emacs-lisp/byte-opt.el

@@ -1288,8 +1288,6 @@ See Info node `(elisp) Integer Basics'."
     form))
 
 ;; Fixme: delete-char -> delete-region (byte-coded)
-;; optimize string-as-unibyte, string-as-multibyte, string-make-unibyte,
-;; string-make-multibyte for constant args.
 
 (put 'set 'byte-optimizer #'byte-optimize-set)
 (defun byte-optimize-set (form)