Deleting a phonological feature could mess up a rule

LT-13357
If one deleted a phonological feature which was used by a phonological rule
as a feature variable and if the feature was the only item in the context,
then the rule would show [???}.  Fixed it so it removes the (now) empty
context.

Change-Id: I140b5b5b1e5e2eecdaac7d805482dcc27ea07ab8