Fix Column dropping operations to use attnum instead of column index

[silentninja]

Fixes #993

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[kgodey]

@silentninja This depends on a draft PR, please mark it as a draft too.

[kgodey]

Looks good to me, please merge when the base PR is merged.

[codecov-commenter]

Codecov Report

Merging #1106 (6e8f695) into master (eb4b5e4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1106   +/-   ##
=======================================
  Coverage   93.38%   93.38%           
=======================================
  Files         112      112           
  Lines        4248     4249    +1     
=======================================
+ Hits         3967     3968    +1     
  Misses        281      281           

Flag	Coverage Δ
pytest-backend	93.38% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
db/columns/operations/drop.py	100.00% <100.00%> (ø)
mathesar/api/db/viewsets/columns.py	91.20% <100.00%> (ø)
mathesar/models.py	96.35% <100.00%> (ø)

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