Language neutral url aliases are not deleted when deleting the parent entity

Problem/Motivation

Followup to #2511968: Path field should fall back to language neutral aliases (also makes this happen for the form widget!)

I just realized language neutral aliases are not removed when deleting parent entty.

Proposed resolution

Adjust PathFieldItemList::delete() in order to remove aliases belonging to non-existing entities.

Remaining tasks

Create test
Create patch
Review

User interface changes

n/a

API changes

n/a

Data model changes

n/a

Release notes snippet