Remove Syntax node from Emacs manual.

Remove Syntax node from Emacs manual.
This node served no purpose in the Customization section.

* doc/emacs/custom.texi (Syntax): Node deleted.

* doc/emacs/emacs.texi (Top): Update node listing.

* doc/emacs/help.texi (Help Summary):
* doc/emacs/major.texi (Major Modes):
* doc/emacs/programs.texi (Parentheses):
* doc/emacs/search.texi (Regexp Backslash, Regexp Backslash)
(Regexp Backslash):
* doc/emacs/text.texi (Words): Callers changed.