cnct: use sweeper in commit resolver

Now that the sweeper is available, it isn't necessary anymore for the
commit resolver to craft its own sweep tx. Instead it can offer its
input to the sweeper and wait for the outcome.

contractcourt/briefcase_test.go

@@ -294,7 +294,6 @@ func TestContractInsertionRetrieval(t *testing.T) {
 			resolved:        false,
 			broadcastHeight: 109,
 			chanPoint:       testChanPoint1,
-			sweepTx:         nil,
 		},
 	}