Remove grype DB diff command

[wagoodman]

As currently written the DB diff command will be cumbersome to keep as a feature while working on v6. We could leave DB diff in tact for v1-5 schemas, but this would be confusing for users attempting to use this diff feature for the latest (v6) DBs.

V6 currently is going to have a blobs table where digests for content could be recomputed and used as a foundation for a diff, however that is out of scope for v6 unless there is a convincing argument / need to keep this feature and add to the v6 scope further.

[TimBrown1611]

hi! @wagoodman diff is important since we want to track changes of the DB.
how can it be done on v6 schema? will v6 contain update date for each record?
thanks!