Merge branch 'eb-remove-flag-reactive-caching' into 'master'

Remove feature flag for reactive caching key check

Closes #29944

See merge request gitlab-org/gitlab!16665

app/models/concerns/reactive_caching.rb

@@ -173,11 +173,7 @@ module ReactiveCaching
     end
 
     def within_reactive_cache_lifetime?(*args)
-      if Feature.enabled?(:reactive_caching_check_key_exists, default_enabled: true)
       Rails.cache.exist?(alive_reactive_cache_key(*args))
-      else
-        !!Rails.cache.read(alive_reactive_cache_key(*args))
-      end
     end
 
     def enqueuing_update(*args)