Remove unnecessary classes

[TimvdLippe]

Looking at https://codecov.io/gh/mockito/mockito/list/e92b0be0c6362157e2c8ae349a2b9d1295065d2e/src and sorting it by coverage, it appears that there are some classes that are completely untested. Taking another closer look reveals some candidate classes that can be removed. For example MockitoLimitations is only used in the Reporter, the field can therefore be placed in the Reporter directly.

There are also some debugging and util classes that seem unused. If we do decide to retain them, I think we should test them to actually make sure they are working properly.