Remove accidental reference to Siren in Core

[alexstandiford]

There's a couple of use statements that reference a Siren namespace. Likely leftover from something I did while building PHPNomad and Siren at the same time.

These should be safe to remove.

core/lib/Abstracts/RemoveDeletedMappingsEvent.php

Lines 9 to 10 in 1884715

	use Siren\Conversions\Core\Models\Conversion;
	use Siren\Mappings\Core\Datastores\Mapping\Interfaces\MappingDatastore;