Delete sqlite metadata support

[dralley]

Background:

It seems the only thing that uses sqlite metadata anymore, is generating repoview files. Except repoview isn't really used anymore and hasn't built since Fedora 28 - it can't be built on EL8 at all without packaging changes. And also, if pulp isn't generating the repoview metadata, then it won't be part of the hosted repo.

The only repo I know of that uses it is RPMFusion, and RPMFusion apparently uses repoview on Fedora 26 just to build these pages. It uses Python 2 and some other ancient dependencies including yum (as a library), which means it can't easily be built on Fedora 28+ / EL8+. And since pulp isn't going to support this anyway, you'd need to be using Pulp just to create the repo, then download the repo and serve it completely separately. You don't really need Pulp to do this, at that point you could just generate the sqlite metadata yourself too.

Rationale: Even though we currently "support" this, it's pretty much untested, untestable and useless, and we would like to reduce unneeded complexity anywhere we can.

Proposal: We announce that this feature will go away in a future release of pulp_rpm and mark the API parameter deprecated. In some future release we would then remove the backing code, and make a decision about whether the API / CLI parameters should just do nothing or be removed entirely.