Remove the "note" database issue status

Summary:
Ref T1191. This was useful for annotating everything but we no longer need it; there are just two types of issues now:

  - Error: stuff we can't fix (missing or surplus tables/database/columns, bad column nullability).
  - Warning: stuff we can fix (column types, character sets, collations, missing or surplus keys, incorrectly defined keys, bad key uniqueness).

Test Plan: Saw 3,399 warnings and 0 errors.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D10594