remove constraint itext id if constraint message is deleted (Case:54143)

If the user clears a constraint message (or hint) text input, clear the
constraint itext id advanced field.

There is a bug when there are multiple language inputs and a user clears
one of them.  The itext id will be deleted even if the other language
itext input still has a value.  But if the user either re-enters a new
value for the language they cleared or clears the other language then it
will end up in the correct state.  Amelia said this is better than the
current behavior.
Merge pull request #41 from dimagi/case-54143

remove constraint itext id if constraint message is deleted (Case:54143)