Remove old RES7 repositories in a database migration

[felixsch]

part of: https://trello.com/c/BSMxlGDN/3166-help-liberty-team-get-suseconnect-working-on-liberty-7

This removes the old obsolete repositories, since RMT does not automatically remove them.

How to test this change:

• Find a RMT server which is running for some time now.
• Check if the server is affected by:

  $ rails c
  > Product.find(1251).repositories.pluck(:name,:scc_id,  :external_url)
  # expect: It should show 4 repositories including 'RES' and 'RES' (src)

• Run the migration:

  $ rails db:migrate
  $ rails c
  > Product.find(1251).repositories.pluck(:name,:scc_id,  :external_url)
  # expect: Does only show 2 repositories

As always, if you have question or suggestions, please do not hesitate to ask!:rocket:

Thank you!

[suse-tests-pass]

Well Done! Your tests are still passing. https://ci.suse.de/job/scc-RMT-integration-tests/355609/console If the given link has expired,you can force a Prophet rerun by just deleting this comment. (Merged c06d61228de16f755b4272679229eaefb41f2c3b into 024478b7c8a9b7c45d617377f2c472917220d603)