Prefer setq-local in reftex

Prefer setq-local in reftex

* lisp/textmodes/reftex-global.el (reftex-find-duplicate-labels)
(reftex-isearch-minor-mode):
* lisp/textmodes/reftex-index.el (reftex-index-mode)
(reftex-display-index, reftex-index-initialize-phrases-buffer)
(reftex-index-phrases-mode):
* lisp/textmodes/reftex-sel.el (reftex-select-label-mode)
(reftex-select-bib-mode, reftex-insert-docstruct, reftex-select-item):
* lisp/textmodes/reftex-toc.el (reftex-toc-mode, reftex-toc):
* lisp/textmodes/reftex.el (reftex-fontify-select-label-buffer):
Prefer setq-local.

(cherry picked from commit b9ce39ba809f49fe80d5b9a6d41afe1fe144fc77)