clean up list_repos_page_size expt

[matt-codecov]

fixes codecov/engineering-team#1757

the default value for users not in this experiment used to be 100 and now the hardcoded value is 50. this is because the experiment group which uses a page size of 50 was rolled out to 100% and the in-code default was just never updated.

[joseph-sentry]

If this merges does this mean we can safely delete the Feature Flag in the Django Admin?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.11%. Comparing base (c9d8b87) to head (a1be492).

❗ Current head a1be492 differs from pull request most recent head 9b581df

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #362      +/-   ##
==========================================
- Coverage   89.75%   89.11%   -0.65%     
==========================================
  Files         376      324      -52     
  Lines       11660     9698    -1962     
  Branches     2065     1735     -330     
==========================================
- Hits        10465     8642    -1823     
+ Misses       1106      992     -114     
+ Partials       89       64      -25     

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[matt-codecov]

@joseph-sentry we could, but i think i would prefer to leave it in case we want to re-run the A/B test results for this experiment later. there is an "inactive" field on the model we can set which currently does nothing but could be made to act as a filter in the UI