Remove ci_yaml_limit_size feature flag

Ref: https://gitlab.com/gitlab-org/gitlab/-/issues/29875

What does this MR do and why?

This MR removes our ci_yaml_limit_size feature flag which has been enabled over 2 year ago.

This flag is currently being used to disable our YAML file size and depth limit.

The workaround for users who still want to disable it is to increase the limit at the application level as explained in our documentation which was introduced in !68593 (merged).

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.