remove parent POM spoon-parent

The semantics of parent POM in Maven is low understood, error-prone and conflicts with proper continuous delivery.

I propose to remove spoon-parent from spoon-core, which simply means copying the required dependencies in the main pom.xml.

Advantages:
* good software engineering with low coupling
* spoon-core does not depend on SNAPSHOTS (see #5395)
* easier CD and testing (fix #5340)

We would first remove it from spoon-core and later for submodules.