Clean up weird "Backlog Management" item on product features page

On both the product feature comparison pages (GitLab.com and self-managed) there is a misfit item called Backlog Management.

It's odd because it's the only item on the page without a "Feature Details" link, and also it's not clear what it represents - it's designated as Premium/Silver tier and has a very long description which seems to incorporate many features.

I'm pretty sure that at least some of those features (like an "organized list which enables the team to prioritize") are actually available at the Core tier, and I'm not honestly sure that any of that is Premium/Silver-tier functionality. If it is, I suspect it has an item of its own.

Really someone in Product Marketing should make the call, but I think the item should either be removed or better defined to reflect the current product state.

/cc @davistye @williamchia @mdelaossa @gweaver @kristmcg