Maintenance.php: Fix mwdebug error

Resolve error with showJobs.php --mwdebug option. Previously, it displayed an error if not followed by "=1".
This was resolved by changing
$this->addOption( 'mwdebug', 'Enable built-in MediaWiki development settings', false, true );
to
$this->addOption( 'mwdebug', 'Enable built-in MediaWiki development settings', false, false );

Bug: T233257
Change-Id: I322fa539a302c2726fffd2420f7f56aec476b32b