Remove remaining calls to MWInit methods

... as well as the require_once statements in includes/WebStart.php
and maintenance/doMaintenance.php, now that the autoloader lists
MWInit (since r85807 / c68957c5e3ada65806294bc9bc5938aa18c6e50c).

Also removed code paths in maintenance/userDupes.inc that seem to be
dead (class_exists( 'Revision' ) should always be true) and useless
global/require_once statements in languages/Language.php.

Follows-up Ic3e769f1fbad4f7ad26dd819406796fee48c6b45.

Change-Id: I48fd6810fdb923b3065ae98024912eb18d394415