Remove unused ColumnsForm

[johannaengland]

Every dashboard has its own setting on how many columns it has

Closes #3243.

I did some digging and this has not been in use since 2016 (c7cebd9)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.79%. Comparing base (237e1b6) to head (f953ba4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3280      +/-   ##
==========================================
+ Coverage   60.76%   60.79%   +0.03%     
==========================================
  Files         606      606              
  Lines       43794    43774      -20     
  Branches       48       48              
==========================================
+ Hits        26610    26612       +2     
+ Misses      17172    17150      -22     
  Partials       12       12              

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[hmpf]

I need to set up nav again to be able to test this, so that'll take a while. Codewise it looks fine though.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud