Finish replacing SGML-based JSX detection with js-mode’s parsing

Finish replacing SGML-based JSX detection with js-mode’s parsing

This removes the last dependency on sgml-mode for JSX-related logic.

* lisp/progmodes/js.el (js-jsx--start-tag-re)
(js-jsx--end-tag-re): Remove.
(js-jsx--looking-at-start-tag-p)
(js-jsx--looking-back-at-end-tag-p): Reimplement using text
properties, using syntax information which ought to be slightly more
accurate than regexps since it was found by complete parsing.