Remove feature flag :reactive_caching_check_key_exists
Problem to solve
In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30456 we fixed a caching issue caused by ReactiveCaching
not supporting nil
values.
The fix was introduced via feature flag reactive_caching_check_key_exists
.
In %12.2 we could remove it after assessing that it's fine.