Cleanup `string-equal-ignore-case' declarations.

Cleanup `string-equal-ignore-case' declarations.

Also, a minor declaration cleanup for other `compare-strings' thin wrappers.
* lisp/emacs-lisp/byte-opt.el (side-effect-free-fns): Remove
  `string-equal-ignore-case', `string-prefix-p', `string-suffix-p'.
(side-effect-and-error-free-fns): Add `proper-list-p' (it already
  was in `pure-fns').
(pure-fns): Remove `string-prefix-p', `string-suffix-p'
  (`string-equal-ignore-case' was missing here).
* lisp/subr.el (proper-list-p): Remove partially duplicate `put's from here.
(string-equal-ignore-case, string-prefix-p, string-suffix-p): Add
  `pure' and `side-effect-free' declarations.
(string-equal-ignore-case): Make inline.