Remove Akeyless Integration feature from Rails and Runner Core codebases

Why this issue ?

• We need to cleanup the codebase of the code related to the ci_akeyless_secret feature
• This needs to be done in both Rails and Runner Core codebases
• Currently, this feature is behind a disabled feature flag in production
• We decided to NOT rollout this feature

References

• #475262 (closed)
• #475262 (comment 2598031692)
• https://gitlab.slack.com/archives/CB2S7NNDP/p1751450552237789
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/feature_flags/gitlab_com_derisk/ci_akeyless_secret.yml
• https://docs.gitlab.com/ci/secrets/akeyless/

Tasks to do

• 1. Disable feature flag on all environments by running in #production channel /chatops run feature set ci_akeyless_secret false --dev --pre --staging --staging-ref --production
• Create an MR that deletes the feature flag and related code and tests
• Create a runner MR that deletes the code related to akeyless_server
• Get the Rails MR reviewed, merged and deployed to all environments
• Get the Runner MR reviewed and merged
• Delete the feature flag from all environments by running in #production channel /chatops run feature delete ci_akeyless_secret --dev --pre --staging --staging-ref --production