Uninstalling a plugin
1. Make db backup.
2. Run the following command to downgrade your database:
bundle exec rake redmine:plugins:migrate NAME=redmine_time_tracker_administrator VERSION=0 RAILS_ENV=production
3. Remove your plugin from the plugins folder: #{RAILS_ROOT}/plugins.
4. Restart Redmine
5. Disable REST web service option at Administration->Settings->API if it is not needed for your purposes (uncheck 'Enable REST web service' checkbox).

If migration command failed, follow next instructions to clean you db:
1. Delete next tables from db:
    rtta_app_requests
    rtta_settings
    rtta_user_states
    rtta_users_data
2. Remove all rows containing 'redmine_time_tracker_administrator' from the table 'schema_migrations'. Most likely it is:
    1-redmine_time_tracker_administrator
    2-redmine_time_tracker_administrator
    3-redmine_time_tracker_administrator
    4-redmine_time_tracker_administrator
