remove maude backend and make java the default

**please review**
Note I am actually removing a call to getPrefixLabel here. The issue is that `syntax KLabel ::= "foo" ` doesn't declare a construct whose KLabel is "foo", it's declaring a KLabel /itself/, so returning "foo" from this method leads to incorrect assumptions in the code later on, such as that the arity of a production is always equal to the arity of KItems whose KLabel comes from the KLabel of that production. In any event, this is all temporary hackiness in the implementation. I believe that in kore, there should never be any productions whose return sort is "KLabel". KLabel declarations like the one above in full K should be syntactic sugar for " `syntax KItem ::= foo(KList)` or `syntax KItem ::= foo(K, K)` depending on whether an arity has been declared. And only KLabels with an arity can heat/cool.