Ignore deletes that do not match

[RubenVerborgh]

Delete operations that do not match any triples should silently succeed instead of erroring.

See nodeSolidServer/node-solid-server#1085

[RubenVerborgh]

From a discussion with @timbl:

It is a critical part of the architecture that patches which delete non existent triples fail.

Otherwise we have no control of concurrent editing

[RubenVerborgh]

Alternative implementation in #299

[jeff-zucker]

There was lengthy discussion on this with the consensus being that an attempt to delete a triple that doesn't exist should return a 409 because it might indicate multiple editors on the same resource, see nodeSolidServer/node-solid-server#1085 (comment)