client: kill .supportsTransactionalUndo()

Usage of supportsTransactionalUndo() was removed from ZODB in 2007 - see
e.g. the following commits:

https://github.com/zopefoundation/ZODB/commit/a06bfc03
https://github.com/zopefoundation/ZODB/commit/e667b022
https://github.com/zopefoundation/ZODB/commit/f595f7e7
...

/reviewed-by @vpelletier
/reviewed-on nexedi/neoppod!8

neo/client/Storage.py

@@ -136,9 +136,6 @@ class Storage(BaseStorage.BaseStorage,
     def supportsUndo(self):
         return True
 
-    def supportsTransactionalUndo(self):
-        return True
-
     def loadEx(self, oid, version):
         try:
             data, serial, _ = self.app.load(oid)