Revert "Add locale to http channel URIs and test that all server requests contain locale."

This reverts commit eeb4e81a396b37cc248b9cc28a881bee1af0e698.
Send locale in POST request's body.
Avoid using real web hosts in testing.
(into bug986521) Bug 993901 - Implement logic to decide when to fetch remotely hosted links
Introduce browser.newtabpage.directory prefix for directory preferences.
Implement directory download and timestamping.
Force directory download on directory source URL preference change.
Attempt directory download on newtab load if sponsored links are shown.
Download directory on init.