Bug 1790312 Part 2 - Remove flex item's normal position snapshot in ReflowChildren(). r=dholbert

ReflowChildren() only cares about the flex item's normal position, so we don't
need to output the relative position from `MoveFlexItemToFinalPosition()`.

By making `aFramePos` const for `MoveFlexItemToFinalPosition`, we can delete
`itemNormalBPos` because `framePos` cannot be changed after calling
`MoveFlexItemToFinalPosition`.

This patch shouldn't change the behavior.

Differential Revision: https://phabricator.services.mozilla.com/D157079