remove ActionDispatch::RequestId middleware

Our custom RequestContextGenerator middleware also sets a request id
header, which is heavily used throughout our infrastructure, so there's
no point in setting a second, unused request id header.

test plan: inspect response headers, there should be an
X-Request-Context-Id header but no X-Request-Id header.

Change-Id: If6964a6aaf20968299cb9e3762343be92822efa2
Reviewed-on: https://gerrit.instructure.com/40355
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>