RFC: Remove Delete methods from DagService

[kevina]

When we switch the blockstore over to use multihashes instead of Cids the Deletes from a Dagservice that is backed by a blockstore will become problematic.

From ipfs/kubo#5510:

The dagservice has a Delete method. This is problematic. For example if you insert a raw and a Protobuf version of the same Node then delete the one of those nodes via the dagservice both will be deleted. We should probably delete these methods. The only place the Delete method seams to be used is in dagutils/utils.go. I have marked two of the three instances with comments.

Does the delete methods serve a useful purpose?