Okay I see where the panel.cfg file refers to the main panel and the radio panel. I assume these are .bmp files somewhere like other aircraft panel.cfg files refer to. But I cannot find "main panel" or "radio panel" anywhere in the entire CFS2 directory. Where would they be?
Make sure you're looking for the correct BMP. The background BMP for the main panel is usually defined under [Window00] like this:
[Window00]
file=
panel_background_640.bmpfile_1024=
panel_background_1024.bmpsize_mm=640
position=7
visible=1
ident=MAIN_PANEL
Note that the above is copied from the FS9 default C182. The BMP files are in the Panel folder alongside their corresponding Panel.cfg. Most 3rd party panels will only have one BMP for the main panel. The Radio_Stack pop-up window will not usually have a background BMP as it doesn't need one. I wouldn't have thought the gunsight you're looking for would have anything to do with this.
I figured out that this particular plane (L-39 in Combat Jet Trainer) has the cockpit bmp's embedded in the gauge (.gau) files.
That's a new one on me. Can you post a screenshot of this panel showing the gunsight you wish to remove?