Remove methods from SpecialPage deprecated in 1.17 and 1.18

Remove methods from SpecialPage deprecated in 1.17 and 1.18

No longer used in core or Gerrit extensions. Some patches have been
submitted to remove the last usages. For details see:
* https://gerrit.wikimedia.org/r/#/q/status:open+topic:replace-deprecated,n,z
* https://gerrit.wikimedia.org/r/#/q/status:merged+topic:replace-deprecated,n,z

- SpecialPage::__call() (deprecated in 1.17)
- SpecialPage::executePath() (deprecated in 1.18)
- SpecialPage::exists() (deprecated in 1.18)
- SpecialPage::file() (deprecated in 1.18)
- SpecialPage::func() (deprecated in 1.18)
- SpecialPage::getGroup() (deprecated in 1.18)
- SpecialPage::getPage() (deprecated in 1.18)
- SpecialPage::getPageByAlias() (deprecated in 1.18)
- SpecialPage::getLocalNameFor() (deprecated in 1.18)
- SpecialPage::getRegularPages() (deprecated in 1.18)
- SpecialPage::getRestrictedPages() (deprecated in 1.18)
- SpecialPage::getTitleForAlias() (deprecated in 1.18)
- SpecialPage::getUsablePages() (deprecated in 1.18)
- SpecialPage::includable() (deprecated in 1.18)
- SpecialPage::init()
- SpecialPage::initAliasList() (deprecated in 1.18)
- SpecialPage::initList() (deprecated in 1.18)
- SpecialPage::name() (deprecated in 1.18)
- SpecialPage::removePage() (deprecated in 1.18)
- SpecialPage::resolveAlias() (deprecated in 1.18)
- SpecialPage::resolveAliasWithSubpage() (deprecated in 1.18)
- SpecialPage::restriction() (deprecated in 1.18)
- SpecialPage::setGroup() (deprecated in 1.18)

Change-Id: I5122f0f23255bc1e8b63a7329a6d1653899b7f59