Tiny bootstrap clean-up

It seems unnecessarily complex that the "X-Drupal-Cache: HIT" and "X-Drupal-Cache: MISS" headers are sent in different bootstrap phases. When they were first added in revision 1.276 in #147310: Implement better cache headers for reverse proxies they were both sent in DRUPAL_BOOTSTRAP_LATE_PAGE_CACHE, but since then they have "drifted". Let's bring them back together.