chore(issues): Remove usage of issue-alert-fallback-targeting flag from the backend

[leeandher]

Requires https://github.com/getsentry/sentry/pull/64965

I believe this will disable the fallthrough for all users, ignoring it even if it was set in the past. Since it's GA there should be no risk, though I believe that means there may be more I can delete 🕵️

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (ee681ae) 81.41% compared to head (5405eb2) 81.41%. Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #64971      +/-   ##
==========================================
- Coverage   81.41%   81.41%   -0.01%     
==========================================
  Files        5254     5254              
  Lines      232302   232294       -8     
  Branches    45603    45601       -2     
==========================================
- Hits       189121   189112       -9     
  Misses      37289    37289              
- Partials     5892     5893       +1     

Files	Coverage Δ
src/sentry/apidocs/examples/project_examples.py	100.00% <ø> (ø)
src/sentry/conf/server.py	88.18% <ø> (ø)
src/sentry/features/__init__.py	100.00% <ø> (ø)
src/sentry/models/projectownership.py	97.66% <100.00%> (-0.07%)	:arrow_down:
src/sentry/notifications/utils/participants.py	91.47% <ø> (-0.07%)	:arrow_down:

... and 8 files with indirect coverage changes