Comply with spec for permissions needed to delete a resource

[michielbdejong]

Basically, until solid/web-access-control-spec#47 gets discussed with a proper decision process, let's undo what was done in #729.

I'm not so fussed about the actual behaviour, I just want to start the process of documenting what we consider the spec (see also solid/solid-spec#174), and on which points we are aware of differences between NSS and the spec.

Wherever such differences exist, if we think that NSS is right, then we should update the spec.
And if we think the spec is right (like in this case), we should (at least create a github issue to) fix NSS.

PS: If anyone is aware of any other points on which NSS is not spec-compliant, then let's create issues for each one of those. We can decide to willingly leave NSS broken, but at least we should have github issues that document how NSS is broken.