Remove reference to removed PropertyLabelResolverTest

In b99916e57ae5a06ba7a4bf768539e5a67f1a9d9c, the class PropertyLabelResolverTest
was removed. This removes its entry in WikibaseLib.classes.php.

Change-Id: I27d40c6fb4509c128c6acb66e508b2a736a56b82
Merge "Remove reference to removed PropertyLabelResolverTest"