Remove UIDNEXT

Summary:
It seems that we're not actually using UIDNEXT for anything. There was an issue reported where Dreamhost email accounts (Dovecot) wouldn't return UIDNEXT in the folder status, causing a crash. So I've removed it.

Relevant issue: https://github.com/inboxapp/inbox/pull/78

Test Plan: sync still works just fine. passes tests.

Reviewers: spang

Reviewed By: spang

Subscribers: emfree

Differential Revision: https://review.inboxapp.com/D332