This fixes performance bar not working with AJAX requests.

Both LabKit and Rails were injecting duplicate X-Request-Id headers into
the response stream, which caused the frontend to be interpret the value
as a comma-separated value (e.g. LdEgVXsp1R4, LdEgVXsp1R4).

The new flow behaves as the following:

1. Workhorse injects X-Request-Id to the inbound HTTP request to Rails
2. Rails looks up this value in its RequestId middleware. Since the
value exists, Rails sets the X-Request-Id response header to this value.
3. Workhorse doesn't append another header in the response.

Distributed tracing should still work fine because Workhorse still
generates the correlation ID.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60111