Don't sync the Gmail Inbox folder.

Summary:
Now that we correctly sync X-GM-LABELS and can quickly fetch changed UIDs for a
given HIGHESTMODSEQ value, we don't actually need to sync the Gmail Inbox
folder separately. This lets us use fewer IMAP connections, generally
removes much complexity, and improves sync performance.

We still order the uid download stack in initial sync such that threads in the
inbox get downloaded first.

Test Plan: Run sync. Watch sync run. Run, sync, run!

Reviewers: khamidou, spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D889