To delete a Masking element, edit the GTS file. GIF video below shows doing it on the Top Right pad (I used bbEdit but, any text editor should work (be sure to use UTF-8 encoding). All I did was Delete the item from the file (then, un-deleted it). Save it to see the changes.
To delete a Masked Trace - there are none produced by Fritzing. However, you can add a graphic line to the GTS file if you play around with graphic software, the file… It’s about loading the GTS into software and drawing the line.
[EDIT] Of course, you can export an SVG/other and Edit it in your favorite Graphics program (draw a Line on the Mask svg where you want the masking…) If you load the mask layer and the copper layer, you can draw the line over the copper trace…
