Remove $wgCentralAuthSilentLogin and $wgCentralAuthUseOldAutoLogin

* $wgCentralAuthSilentLogin was removed in 26e4affc9311.
* $wgCentralAuthUseOldAutoLogin was removed in efbdc700188f.

Change-Id: I3ea697b6a5a06dca5d374bbc03468c370b94b3fd