Remove the required Drupal core version from the composer.json file

As documented in New 'core_version_requirement' key in info.yml files for modules, themes and profiles allows Composer semantic version constraints including specifying multiple major versions of core, it is not necessary to have a drupal/core requirement in the composer.json file used by contributed modules: One will be automatically generated based on the core_version_requirement value.

The drupal/core requirement can be removed from the composer.json file.