Remove "disable_ssh_key_used_tracking" feature flag

Vasilii Iakliushinrequested to merge
vi_remove_disable_ssh_key_used_tracking_ff into master

What does this MR do and why?

Contributes to gitlab-com/gl-infra/scalability#889

The feature flag was introduced to resolve the incident. Later, the original problem was fixed in #324764 (comment 1393856569).

The feature flag is not necessary and it should be safe to remove it.

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