Remove "level" fields from pools table in database.

[ian-deans]

Fixes #1017.

Overview

• Add database migration to remove the 'levels' field from the 'pools' table.

Data Model / DB Schema Changes

• 'Pools' table no longer contains unnecessary field 'levels'.
• Will need to run upwards migration.

Environment / Configuration Changes

none

Notes

[ian-deans]

@prattsj, got rid of the code for the down migration and replaced it with the request comment.