Don't define Ansible dependency roles twice

[wsmirnow]

As the Ansible depencency roles are defined in meta/main.yml, we don't need defining them twice in requirements.yml.

How to test:

1. Change to an empty directory
2. Create and enable python venv
3. Install Ansible via pip
4. Make sure Ansible role elan.opencast_repository is not installed
5. Create requirements.yml with following content

---
roles:
  - name: elan.opencast_opensearch
    src: https://github.com/elan-ev/opencast_opensearch
    version: ansible-galaxy-should-install-dependencies

1. Install Ansible roles with ansible-galaxy install -r requirements.yml -f

The elan.opencast_repository Ansible role should also be installed.