Use delete_by_query to remove all duplicate favorite documents

[oalders]

Prod has cases where duplicate favorite documents exist for the same
(user, distribution) pair with different authors -- I don't know exactly
why this is. The previous delete handler used get(), which computes a
single composite ID, deletes one document and leaves orphans behind.
This is possibly why users sometimes reported removing a ++ but not
actually being able to delete it. Switch to search + delete_by_query to
remove all matching documents and purge cache keys for every affected
author.

[oalders]

I should note that I'm not sure why author keys need to be purged, but the previous code already did this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.22%. Comparing base (0a7f2f5) to head (93cb804).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1458      +/-   ##
==========================================
+ Coverage   62.16%   62.22%   +0.06%     
==========================================
  Files         137      137              
  Lines        4393     4400       +7     
  Branches      659      658       -1     
==========================================
+ Hits         2731     2738       +7     
- Misses       1410     1412       +2     
+ Partials      252      250       -2     

Files with missing lines	Coverage Δ
lib/MetaCPAN/Server/Controller/User/Favorite.pm	92.85% <100.00%> (+16.66%)	⬆️

... and 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.