[MergeDups] Track deleted senses

[imnasnainaec]

Follows #2907
Precedes #2893

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This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.76%. Comparing base (3312b57) to head (5a808ba).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2953   +/-   ##
=======================================
  Coverage   72.75%   72.76%           
=======================================
  Files         264      264           
  Lines       10201    10202    +1     
  Branches     1196     1196           
=======================================
+ Hits         7422     7423    +1     
  Misses       2429     2429           
  Partials      350      350           

Flag	Coverage Δ
backend	82.78% <ø> (ø)
frontend	63.40% <100.00%> (+<0.01%)	⬆️

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[jasonleenaylor]

Reviewed 3 of 4 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)