DEV: Remove `cache` option from `ajax()` (PR #13142)

  1. It defaults to cache: true already
  2. Setting it to false for non-GET request doesn’t do anything
  3. We were correcting cache: false GET requests to use cache: true

…so setting it to anything at all, for any type of request doesn’t make sense (anymore)


Maybe wait until next week to merge just in case?