Remove CI header presence check under FF

What does this MR do and why?

This commit adds a ci_remove_header_presence_check feature flag.

When enabled, it removes the conditional that skips interpolation if the content has no header for CI components (all other include types will still skip interpolation if there is no header).

We will slowly enable the feature flag and monitor the performance of include:component. This will let us know if we can add the new context interpolation feature without hurting performance too much. Performance will be monitored here (rollout issue will contain the updates): https://log.gprd.gitlab.net/app/r/s/NA84W

Next steps, in order:

If all goes well with performance, we can clean up the feature flag that will is allowing the interpolation to happen without any headers.

1. Issue for this MR: #502404 (closed)
2. Rollout issue: #474888 (closed)
3. We can unblock the feature issue: #438275 (closed)