remove maude backend and make java the default

**please review**
LGTM (and yes, this code will change soon, hopefully, because this is just a deconstructor for a KORE syntactic constructor; the plan is that the KORE library will provide implementations for all the constructors and the associated deconstructors).

Syntactically, KORE does not restrict you to declare `syntax KLabel ::= "foo"`.  But we are going to have all sorts of additional restrictions that will be checked and enforced after parsing.  Not sure yet whether this will be one of them or not, but we do not have to decide it now.