Revert "masks: fix bug in last commit"

This reverts commit df0e4c63c780884cd58131a043f443e77e60670d.

this and the preceding commit lead to crashes. we need to find an alternative
to fix the issue with deleting shapes.
Revert "masks: correctly initialize form_gui->edit_mode: fixes regression when removing shapes"

This reverts commit 2eff9bd237561f33ac8f8370e2f1d5bbd4f832a9.

as before