deprecate nltk.classify.svm

All code from nltk.classify.svm is removed in this PR without providing a compatibility layer. 

`nltk.classify.SvmClassifier` is no longer available (to not confuse people when `dir(nltk.classify)` is called). `nltk.classify.svm.SvmClassifier` is still there, but raises an exception on instantiation.

See GH-450, GH-170 and GH-261