Empty the migration which fixed MySQL collation

This migration was used to fix some collation inconsistencies in
MySQL but we don't need its functionality on PostgreSQL. The migration
will remain in the schema_migrations table in the database, so we're
leaving this file in place but with empty methods to avoid the
potentially confusing '********** NO FILE **********' message shown by
`rake db:migrate:status`.
Merge pull request #403 from alphagov/postgres-empty-collation-migration

Empty the migration which fixed MySQL collation