Revert "Distinguish common and advanced macros/envs."

This reverts commit f8a3a8bdd4aae28d302b5c04fa602584a7406f4c.
void-function error fix.

* latex.el (LaTeX-common-initialization): Fix void-function
LaTeX-symbol-list error.
Distinguish normal and expert macros/envs.

* tex.el (TeX-complete-expert-commands): New defcustom.
(TeX-complete-make-expert-command-functions): New macro.
(TeX-insert-macro): Restrict completion depending on
`TeX-complete-expert-commands'.
(VirTeX-common-initialization): Ditto.

* latex.el (LaTeX-environment, LaTeX-common-initialization):

* tex-info.el (TeX-texinfo-mode): Restrict completion depending on
`TeX-complete-expert-commands'.

* doc/auctex.texi: Document normal vs. expert commands for users
and style file authors.

* doc/changes.texi: Mention normal vs. expert commands.
Better signature of expert declaration functions.

* tex.el (TeX-complete-make-expert-command-functions): Change
signature of declare-expert functions.

* doc/auctex.texi: Document it.