Remove the JUnit 4 MockitoComponentManagerRule

The JUnit 4 MockitoComponentManagerRule has no user left in XWiki Commons, Rendering and Platform (its last user, MockitoOldcoreRule, is removed in XWIKI-25438). JUnit 5 tests use @ComponentTest / MockitoComponentManagerExtension instead.

Remove it, as a step towards removing JUnit 4 (and the JUnit Vintage engine) from the test classpath altogether. Extensions still having JUnit 4 tests using it need to migrate them to JUnit 5.