chore(feedback): rm spam UI feature flag

[michellewzhang]

closes https://github.com/getsentry/sentry/issues/70346

[codecov[bot]]

Codecov Report

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

Project coverage is 80.01%. Comparing base (48d9c5c) to head (557b19b). Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #70359   +/-   ##
=======================================
  Coverage   80.01%   80.01%           
=======================================
  Files        6505     6505           
  Lines      290794   290793    -1     
  Branches    50121    50121           
=======================================
  Hits       232684   232684           
+ Misses      57673    57672    -1     
  Partials      437      437           

Files	Coverage Δ
src/sentry/conf/server.py	89.42% <ø> (ø)
src/sentry/features/temporary.py	100.00% <ø> (ø)

... and 7 files with indirect coverage changes