Revert "Add method Model::getDatabase()."

This reverts commit 31cc85b7473271130438f6ab29e99e2a90f16cb0.
Revert "Pass database connections to the result."

This reverts commit 6452e127fb0e54221281df2ce8f351b33248aadd.

Conflicts:
	system/modules/core/library/Contao/Model.php
Revert "Add a registry to each database connection."

This reverts commit 4b11dbe3cf7ff1fec6aeb53599e11ae92930a421.
Make the model registry a singleton.