Revert "Cache 4xx responses on the mobile backend servers for 5m"

This reverts commit bef86cbb3d7efa13c95481148de2f6a561845f08

This made 403s emitted by MediaWiki with Cache-Control: no-cache be cached for
5m. In particular, checkers.php's check for empty user-agent script made the
403 response to be cached for all users for 5m.

Change-Id: I9400eeae70c91e146bebcebcf45742f9697d3b58
Merge "Revert "Cache 4xx responses on the mobile backend servers for 5m"" into production