Keep nxml prolog updated via syntax-propertize

Keep nxml prolog updated via syntax-propertize

Instead of using after-change-functions.  Also, stop consulting
nxml-prolog-regions during syntax-propertize.  It turns out the
problems fixed by using prolog information are actually due to using
the wrong syntax table during propertizing.  This was fixed in
2019-06-04 "* lisp/emacs-lisp/syntax.el: Use syntax-ppss-table for
syntax-propertize." so consulting the prolog data is no longer needed.
* lisp/nxml/nxml-rap.el (nxml-maybe-rescan-prolog): Remove.
* lisp/nxml/nxml-mode.el (nxml-mode): Stop using it.
(nxml-syntax-propertize): Don't use nxml-prolog-regions, just call
nxml-scan-prolog if needed before delegating to
sgml-syntax-propertize.
* test/lisp/nxml/nxml-mode-tests.el (nxml-mode-edit-prolog): New test.