Remove the clearfix helper

[andresgalante]

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

Considering that Bootstrap no longer supports old IE, nor does grids with floats, is the clearfix helper still necessary?

If not, should we just remove it to reduce the bundle size?

Motivation and context

This is small, but deleting unnecessary code is always good :)

[mdo]

Dropping it in v6 for display: flow-root utility.