Remove position: relative from grid columns

Prompted by #25254, I dug into our Git history to see why we add `position: relative` to our grid classes and mixins. Turns out, I have no idea and added it to v3 in a commit that fixed an unrelated issue—see 0f17f9b.

scss/mixins/_grid.scss

@@ -28,7 +28,6 @@
 }
 
 @mixin make-col-ready() {
-  position: relative;
   // Prevent columns from becoming too narrow when at smaller grid tiers by
   // always setting `width: 100%;`. This works because we use `flex` values
   // later on to override this initial width.