Proper delete of close bracket for mixset inside method

[gublan24]

The current implementaion to handle mixsets inside methods does not remove the proper closing bracket of a mixset when it is nested in a complicated method structure. The generated code from the provided test case (in current Umple version) will generate code containing:

//TAG_MIXSET_BEFORE_CLOSE
}  
// The above closing bracket should be removed as StateMachine is used.
//TAG_MIXSET_CLOSE

This PR improves handling mixsets inside method bodies by removing the proper close brackets.