Get rid of wikipedia logo

This was being used as the icon for the welcome and user rights notifications,
but is completely inappropriate for any site other than Wikipedia.

WMF will probably want something like this in their config:
if ( $site === 'wikipedia' ) {
	$wgEchoNotifications['welcome']['icon'] = 'w';
	$wgEchoNotifications['user-rights']['icon'] = 'w';
}

Bug: 47650
Change-Id: Idd41567ffcb468dd3d040d2a5a61d1e3f9f7f5fa
Merge "Get rid of wikipedia logo"