FIX: Removed cheatManyList

The functionality of the GridFieldManyRelationHandler_ManyManyList
exists at least in 3.1-beta2, this causes problems as there are now
different signatures of the getters.
I therefore removed the GridFieldManyRelationHandler_ManyManyList-class and all references to it, using the new built-in-calls to the attributes of the ManyManyList...

Best regards,
s-m