Improve DeleteSubjectJob when run as DeferredJob

I wasn't really satisfied on how the deletion job was forced to handle
historic data, where it solely depended on the Store's data fetch at the time
of execution and withit did influence as to when to schedule the JobQueue.

Luckily, since SMW 1.9 we have a SemanticData serializer/deserializer
which enables to serialize SemanticData and can be used before the subject and
its data are removed.

Running [1] the serializer adds a diminutive overhead during the deletion
process which enables to delete a subject immediately without delay but
schedule the update associative entities as deferred jobs as it has a lower
priority and can be scaled in the background.

[1] 'smwgDeleteSubjectWithAssociatesRefresh' => true