Revert "Change default collapse function to sum"

[javerbukh]

Reverts #1229

[codecov]

Codecov Report

Merging #1236 (a54c310) into main (d08a4ea) will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1236      +/-   ##
==========================================
- Coverage   77.93%   77.84%   -0.09%     
==========================================
  Files          90       90              
  Lines        7205     7203       -2     
==========================================
- Hits         5615     5607       -8     
- Misses       1590     1596       +6     

Impacted Files	Coverage Δ
jdaviz/configs/specviz/plugins/viewers.py	80.24% <ø> (-0.16%)	⬇️
...configs/default/plugins/data_tools/file_chooser.py	65.71% <0.00%> (-3.43%)	⬇️

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[pllim]

Superseded by #1237