Remove highlight_diffs_optimize_memory_usage feature flag

What does this MR do and why?

The feature flag has been introduced in !92456 (merged) to set a smaller TTL for highlighted diffs cache. The feature flag has been 100% enabled. The hit ratio dropped at the beginning because the key of the cache has been changed as well, but after some time it normalized to the previous level:

Related issue: #367160 (closed)