Remove focus & hover states from `bg-variant` mixin

Created by: MartijnCuppens

About this code: https://github.com/twbs/bootstrap/blob/8896827bd072a120705a0576a5bcde69c8ce2cda/scss/mixins/_background-variant.scss#L9-L14

I think we should remove it. IMO focus and hover background states don't belong in these utilities and we are also not defining an active state here. We have the buttons with backgrounds, if this doesn't cover the needs I think the developer will need to write custom code, because it's quite specific.

I guess we can just remove the mixin, just use the background-color property in v5 and use the deprecate mixin to deprecate this in v4.

Thoughts?