forbid extending sort K in the Java backend

Fixed https://github.com/kframework/k/issues/708

@dwightguth @radumereuta please review
Maybe we should do the same thing in both java and maude backends.
I always assumed that K is the base sort for all syntax, but since the java backend was introduced, it seems KItem is the preferred sort.
Maybe we should look over all the definitions, and I could go through the SDF stuff and fix all these inconsistencies.