Remove the Filter action from DB editor

[soininen]

This removes the Filter icon, the associated QAction and all related code from DB editor. The action was redundant and didn't work very well.

Resolves #2820

Checklist before merging

• Documentation is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.12%. Comparing base (9a5c21b) to head (5aa424e).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3007      +/-   ##
==========================================
- Coverage   67.15%   67.12%   -0.04%     
==========================================
  Files         193      193              
  Lines       37138    37011     -127     
  Branches     4450     4443       -7     
==========================================
- Hits        24940    24843      -97     
+ Misses      10941    10913      -28     
+ Partials     1257     1255       -2     

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[manuelma]

I wish we could have fixed this instead of deleting it, but maybe for another day. The filter action was great to see exactly what the tool was getting after applying the filter.