Removed deprecated $wgDisabledActions

$wgDisabledActions is deprecated since 1.18 and is hardly used
anywhere. It occurs once in includes/DefaultSettings.php where
it is initialized to an empty array.And in Setup.php, for every
action in $wgDisabledActions $wgActions is set to false. Also
added Release notes

Bug: 60460
Change-Id: I5fab95c76b2742b12c1f397d27898148625a9959