[thci] remove unused methods (#7704)

`setActiveDataset` and `setCommisionerMode` have been removed from
the `iThci` abstract class in the latest harness release.

tools/harness-thci/OpenThread.py

@@ -2740,16 +2740,6 @@ class OpenThreadTHCI(object):
 
         return self.__executeCommand(cmd)[-1] == 'Done'
 
-    @API
-    def setActiveDataset(self, listActiveDataset=()):
-        # Unused by the scripts
-        pass
-
-    @API
-    def setCommisionerMode(self):
-        # Unused by the scripts
-        pass
-
     @API
     def setPSKc(self, strPSKc):
         cmd = 'dataset pskc %s' % strPSKc