Remove drupal/core requirement from composer.json

Problem/Motivation

From https://www.drupal.org/docs/develop/using-composer/add-a-composerjson-fi...

If your project does have a composer.json file, having a drupal/core version requirement is also not required for Drupal 9 compatibility. It is better not to provide a drupal/core version requirement in composer.json because Drupal's composer facade will generate the appropriate metadata based on the info.yml file.

And speaking as a site maintainer, modules that include core as a dependency can be a little troublesome if you have to do something weird while updating dependencies.

Proposed resolution

Delete the requirement.

Remaining tasks

Delete the requirement.