Remove feature flags related to diff highlight expiration
What does this MR do and why?
Now the TTL of highlighted diff cache is 1 hour. The feature flags have been enabled for a while now. Since they were introduced to safely reduce TTL on Gitlab Sass, we can avoid the step of enabling them by default and just remove them.
Closes: #370728 (closed)
Related issue: #367160 (comment 1088250584)