Revert "DEV: Define --footer-nav-height css var (#14008)" (PR #14327)

This reverts commit 3119b881aa6c297e3fdb281188a1d1b9a5fcd349.

This caused the nav bar to sometimes show up in the middle of the viewport. I believe this is because we use -$footer-nav-height in this file. That translates to -var(--footer-nav-height) in the outputted CSS and it seems to cause issues.

I checked with @markvanlan as well, we no longer use this property in a plugin, so it’s safe to revert.