Remove deprecated $wgSpecialPageGroups

Remove deprecated $wgSpecialPageGroups

$wgSpecialPageGroups is deprecated since 1.21
override SpecialPage::getGroupName instead

Remove also SpecialPageFactory::setGroup because it no long can set $wgSpecialPageGroups.
Also remove SpecialPageFactory::getGroup along with its setter.

All replaces in extensions can be found under:
https://gerrit.wikimedia.org/r/#/q/status:open+branch:master+topic:wgSpecialPageGroups,n,z
https://gerrit.wikimedia.org/r/#/q/status:merged+branch:master+topic:wgSpecialPageGroups,n,z

Change-Id: I1aa8f98bf326b2e54d7403efbb9f002a106cefc5