Remove Mojo test cases

Tycho currently contains some so called "Mojo Testcases" that are ones
that (partly) simulate the execution of a mojo, but these have some
problems:

1) They currently depend on heavy outdated (and deprecated)
maven-plugin-testharness
2) They often do not have a full test setup so we need to have some
quirks in the code (e.g components are null)
3) They are often hard to understand as they are a mixture of
(incomplete) maven setups with things setup in the code
4) We even have some constructors only used to support these cases
5) As they often test very specific internal behavior they often need
adjustments.
6) For the Maven 4 transition one needs to adjust them to new maven4 api
and components and executions.

On the other hand we have a very good integration suite that is made of
actual executable examples and test, changing internal implementation
details does not affect these or show clear use-cases that break.

Because of this and to prepare for the Maven 4 transition these tests
are now deleted.