Fix a crash when ungrouping a "maskhelper" group

This MR fixes a segfault reported in #3616 (closed) which occurs when the user has clip set on a group with the attribute inkscape:groupmode="maskhelper" and decides to dissolve the group manually via the "Ungroup" action.

Since the ungroup operation can now preserve masks and clips, it must temporarily unset them on the group before it sets them on the children of the group. The trouble is that the code responsible for the removal of masks can detect "maskhelper" groups and remove them automatically, which means that Inkscape would try to delete the same group twice: once during the temporary mask removal, and then again during the actual delete step.

Fixes #3616 (closed)