Remove implicit association support from code generator

[Matthias Schoettle]

Currently, the code generator contains logic to generate fields with multiplicity many of type List and imports java.util.* by default.

Once the new Association concern is integrated into master, this logic should be removed.

We should also consider removing the special collection (RSequence and RSet I believe) types from the RAM metamodel.

[Matthias Schoettle]

Might be possible to do for someone who wants to work on the code generator more.

[Matthias Schoettle]

Resolves #427: Removes RSet and RSequence usage.

Previous commit already removed the default java.util.* import.

→ <<cset 202c2538ee07>>