feat(notifications): remove notification double write feature flag

[scefali]

Now that we have double writes enabled for everyone, we can remove the feature flag entirely

[codecov]

Codecov Report

Merging #57863 (4f47681) into master (de3a4e8) will increase coverage by 0.66%.
Report is 12 commits behind head on master.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master   #57863      +/-   ##
==========================================
+ Coverage   78.39%   79.06%   +0.66%     
==========================================
  Files        5135     5137       +2     
  Lines      223461   223577     +116     
  Branches    37624    37638      +14     
==========================================
+ Hits       175185   176760    +1575     
+ Misses      42629    41167    -1462     
- Partials     5647     5650       +3     

Files	Coverage Δ
...dpoints/organization_events_root_cause_analysis.py	96.19% <100.00%> (+0.07%)	⬆️
...try/api/endpoints/user_notification_fine_tuning.py	100.00% <100.00%> (+1.16%)	⬆️
src/sentry/conf/server.py	89.83% <ø> (ø)
src/sentry/features/__init__.py	100.00% <ø> (ø)
src/sentry/notifications/helpers.py	91.07% <100.00%> (-0.49%)	⬇️
src/sentry/notifications/manager.py	85.41% <100.00%> (+0.86%)	⬆️
...sentry/services/hybrid_cloud/notifications/impl.py	76.85% <100.00%> (+0.70%)	⬆️
static/app/components/avatar/avatarList.tsx	90.90% <ø> (ø)
...c/sentry/integrations/slack/views/link_identity.py	86.36% <50.00%> (ø)
...ns/notifications/activity/new_processing_issues.py	89.79% <50.00%> (ø)
... and 1 more

... and 162 files with indirect coverage changes