lisp/nxml: Use syntax-tables for comments

lisp/nxml: Use syntax-tables for comments

* lisp/nxml/nxml-mode.el (nxml-set-face): Prepend.
(nxml-mode): Set syntax-ppss-table.
Use sgml-syntax-propertize-function for syntax-propertize-function.
Let font-lock highlight strings and comments.
(nxml-degrade): Don't touch "nxml-inside" property any more.
(nxml-after-change, nxml-after-change1): Remove functions.
(comment): Don't set fontify rule any more.
(nxml-fontify-attribute): Don't highlight the value any more.
(nxml-namespace-attribute-value-delimiter, nxml-namespace-attribute-value)
(nxml-comment-delimiter, nxml-comment-content): Remove faces.

* lisp/nxml/nxml-rap.el (nxml-scan-end): Remove.
(nxml-get-inside, nxml-inside-start, nxml-inside-end): Use syntax-ppss.
(nxml-clear-inside, nxml-set-inside): Remove.
(nxml-scan-after-change): Remove function.
(nxml-scan-prolog, nxml-tokenize-forward): Simplify.
(nxml-ensure-scan-up-to-date): Use syntax-propertize.
(nxml-move-outside-backwards):
* lisp/nxml/nxml-outln.el (nxml-section-tag-backward): Adjust to new
nxml-inside-start behavior.

* lisp/nxml/nxml-util.el (nxml-debug-set-inside)
(nxml-debug-clear-inside): Remove macros.

* lisp/nxml/xmltok.el (xmltok-forward-special): Remove function.
(xmltok-scan-after-comment-open): Simplify.