Term::postDelete() needlessly uses entity_delete_multiple()

Problem/Motivation

\Drupal\taxonomy\Entity\Term::postDelete() calls out to entity_delete_multiple() even though it already gets the respective storage handler - that actually takes care of the deletion in the end - passed in.

Proposed resolution

Use the storage handler directly.

Conrecetely:

1. Instead of fetching the IDs of the children to build of the $orphans variable, use the $child variable directly
2. Instead of calling entity_delete_multiple() call the delete() method on $storage passing the $orphans variable.

Note: This could theoretically go into 8.1.x to limit code divergence, but not sure, so tagging against 8.2.x.

Remaining tasks

Write patch.

User interface changes

None.

API changes

None.

Data model changes

None.