Remove vc-header-alist, obsolete since 21.1.

Remove vc-header-alist, obsolete since 21.1.

* lisp/vc/vc-hooks.el (vc-header-alist): Remove obsolete variable.
* lisp/vc/vc.el (vc-static-header-alist): Doc fix.
* lisp/vc/vc-cvs.el (vc-cvs-header):
* lisp/vc/vc-rcs.el (vc-rcs-header):
* lisp/vc/vc-sccs.el (vc-sccs-header):
* lisp/vc/vc-svn.el (vc-svn-header): Do not consult vc-header-alist.
* lisp/obsolete/vc-mcvs.el (vc-mcvs-header):
* lisp/progmodes/cperl-mode.el (cperl-mode): Only set vc-header-alist
on XEmacs.

* etc/NEWS: Mention above change.