ref(similarity): remove backfill endpoint

[JoshFerge]

this code can be removed as for future backfills we'll use getsentry jobs.

[codecov]

Codecov Report

Attention: Patch coverage is 86.20690% with 16 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/seer/autofix_tools.py	38.88%	11 Missing ⚠️
tests/sentry/seer/test_autofix.py	91.52%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #87448       +/-   ##
===========================================
+ Coverage   33.16%   87.76%   +54.59%     
===========================================
  Files        8358     9887     +1529     
  Lines      466166   560745    +94579     
  Branches    22076    22075        -1     
===========================================
+ Hits       154620   492112   +337492     
+ Misses     311142    68229   -242913     
  Partials      404      404