Remove composer.json

As the module is only depending on Drupal core and Drush, there is no need to have a composer.json file. Removing it while avoid to have to maintain it.

For example, currently, it requires Drush 9. All projects with Drush 10 won't be able to install the module.

Only use composer.json if you are requiring third-party libraries.

Apart from that drupal.org will create the correct composer.json automatically for every release.