ref: #376 Only the last commit is relevant (will rebase out after) but I built from my previous branch since I think the extraction helps readability. Basically JS-Data already goes down the removeInverseRelation pathway when a relation is set to null/undefined so unlinkInverseRecords is strictly unnecessary.

Further the code in unlinkInverseRecords is flawed (or it would only work correctly for hasOne:

utils.set(record, this.getInverse(this.mapper).localField, undefined)

Compare to (removeInverseRelation):

else if (inverseDef.type === hasManyType) {
  ...
 utils.remove(children, (child) => child === this || id === utils.get(child, idAttribute))

Unfortunately simply removing removeInverseRelation broke a test in the destroy suite. This is because the hasOne setter() does not clear correctly, so this was updated as well (see below).