Remove the upload letters permission

As of https://github.com/alphagov/notifications-admin/pull/3690 it’s no
longer referred to.

migrations/versions/0317_uploads_for_all.py

@@ -6,12 +6,12 @@ Create Date: 2019-05-13 10:44:51.867661
 
 """
 from alembic import op
-from app.models import UPLOAD_LETTERS
-
 
 revision = '0317_uploads_for_all'
 down_revision = '0316_int_letters_permission'
 
+UPLOAD_LETTERS = 'upload_letters'
+
 
 def upgrade():
     op.execute("""