Change default LockManager back to nullLockManager.

* This was changed in 926db7c3bd as a workaround for a testing issue.
* See 8cbd679aac781bdd73038064261736a937645fc1 , which fixes the test harness in core .

Change-Id: Ib57cafee35a699ce875ee4bc8951781e1a37553a

MathTexvc.php

@@ -232,7 +232,7 @@ class MathTexvc extends MathRenderer {
 			if ( !$backend ) {
 				$backend = new FSFileBackend( array(
 					'name'           => 'math-backend',
-					'lockManager'    => 'fsLockManager',
+					'lockManager'    => 'nullLockManager',
 					'containerPaths' => array( 'math-render' => $wgMathDirectory ),
 					'fileMode'       => 0777
 				) );