Clean up WikimediaMaintenance class

The code in WikimediaMaintenance::loadSettings() was superseded by
equivalent code in multiversion/MWScript.php. $wgNoDBParam has been
replaced by the $wikiless list there. WikimediaMaintenance was always
the wrong place for --wiki argument parsing, since the --wiki argument
is meant to work even for core maintenance scripts. The $site and $lang
globals and the wikilang environment variable are no longer read by
anything. The TESTWIKI constant is set by MWVersion.php.

So basically, the only effect of this class was to screw up attempts to
run these maintenance scripts on non-WMF installations.

Change-Id: Icdd0f89b24b860d84e041acba51d7b1931065852