htlcswitch+peer: remove ShutdownIfChannelClean

peer/test_utils.go

@@ -485,9 +485,6 @@ func (m *mockUpdateHandler) EligibleToForward() bool { return false }
 // MayAddOutgoingHtlc currently returns nil.
 func (m *mockUpdateHandler) MayAddOutgoingHtlc(lnwire.MilliSatoshi) error { return nil }
 
-// ShutdownIfChannelClean currently returns nil.
-func (m *mockUpdateHandler) ShutdownIfChannelClean() error { return nil }
-
 type mockMessageConn struct {
 	t *testing.T