Remove backwards compatibility layer in GroupSubscribeForm

[pfrenssen]

Do not work on this before Drupal 8.6.0 is released, this is planned for September 2018

In Drupal 8.6.0 a change was made to the ContentEntityForm class: one of the constructor arguments was changed from EntityManagerInterface to EntityRepositoryInterface.

In #395 a workaround was added in GroupSubscribeForm (which extends ContentEntityForm) to ensure that the form is compatible both with Drupal 8.6 and earlier versions without throwing deprecation warnings.

Once Drupal 8.5.x is deprecated we do not need this B/C layer any more and it can be removed.

• Add the EntityRepositoryInterface type hint to the $entity_repository argument in GroupSubscribeForm::__construct(), and update the docblock.
• Remove the B/C layer from GroupSubscribeForm::create() and update the documentation.
• Update the version requirement of Drupal core to ~8.6 in composer.json.