Do no longer manage version of JMock nor any other mocking framework

Currently a pretty outdated version of JMock is managed in the parent pom.xml (https://github.com/apache/sling-parent/blob/96e3aa372ae124d0ab982d9c3fe560766f2d5ede/sling-parent/pom.xml#L1024).

In different Sling modules different mocking libraries are used but IMHO Mockito has probably the most usages. As the mocking frameworks are not used consistently I would suggest to no longer manage any versions of mocking frameworks in parent.