multi: remove returned error from WipeChannel

The linter complains about not checking the return value from
WipeChannel in certain places. Instead of checking we simply remove the
returned error because the in-memory modifications cannot fail.

htlcswitch/mock.go

@@ -602,9 +602,7 @@ func (s *mockServer) AddNewChannel(channel *channeldb.OpenChannel,
 	return nil
 }
 
-func (s *mockServer) WipeChannel(*wire.OutPoint) error {
-	return nil
-}
+func (s *mockServer) WipeChannel(*wire.OutPoint) {}
 
 func (s *mockServer) LocalFeatures() *lnwire.FeatureVector {
 	return nil