Drop cache column from repos table

[joseph-sentry]

Purpose/Motivation

The cache column is being removed to an issue where it becomes corrupted and it is impossible to run any queries on the row where the column is corrupted.

I think this actually needs to get merged first (this should probably be merged already): #53

Links to relevant tickets

codecov/engineering-team#92

What does this PR do?

• Remove cache field from Repository model
• Remove cache from read_only_fields in RepositoryAdmin
• Create migration for the above using makemigrations

[scott-codecov]

Trent reminded me that we're still running https://github.com/codecov/codecov.io in production to serve some old public API v1 endpoints. We'll need to make sure those don't also rely on repos.cache in any way.

In the meantime I wonder if it's possible to remove the field from the model definition but keep the column for now? This would mean Django would stop querying it but leave codecov.io working as-is. I'm honestly not sure if that's possible with Django - seems like it should be.

[codecov-staging]

Codecov Report

Patch coverage is 100.00% of modified lines.

Files Changed	Coverage
core/admin.py	ø
core/models.py	ø
core/migrations/0033_remove_repository_cache.py	100.00%

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[codecov]

Codecov Report

Merging #80 (bc3104c) into main (5308c45) will not change coverage.
The diff coverage is 100.00%.

@@          Coverage Diff          @@
##            main     #80   +/-   ##
=====================================
  Coverage   95.26   95.26           
=====================================
  Files        695     696    +1     
  Lines      14776   14780    +4     
=====================================
+ Hits       14075   14079    +4     
  Misses       701     701           

Flag	Coverage Δ
unit	95.20% <100.00%> (+<0.01%)	⬆️
unit-latest-uploader	95.20% <100.00%> (+<0.01%)	⬆️

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

Files Changed	Coverage Δ
core/admin.py	76.27% <ø> (ø)
core/models.py	94.53% <ø> (-0.03%)	⬇️
core/migrations/0034_remove_repository_cache.py	100.00% <100.00%> (ø)