remove unconnected contextMenuEvent

[jo-mueller]

Closes #456

Hi @zoccoler , I think this fixes the error you saw. In essence, I removed the previous functionality for exporting subclusters with a right-click triggered context menu but forgot to remove the callback to the right click itself.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.55%. Comparing base (e5e7444) to head (3ceec63).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #458      +/-   ##
==========================================
+ Coverage   85.50%   85.55%   +0.05%     
==========================================
  Files          11       11              
  Lines        1276     1274       -2     
==========================================
- Hits         1091     1090       -1     
+ Misses        185      184       -1     

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