`Pane::_FindFocusAndNeighbor` can probably be simpler

If we view this problem from the point of the focused pane, what this function (and the other) do is basically:

• move to the parent node
• check if the other child node - the one the focused pane doesn't belong to - is in the direction we want to move the pane to
  • if yes: swap panes
  • if no: continue with the parent node

Is that correct? If my understanding is correct, I believe we can simplify this code after it has been merged.

Originally posted by @lhecker in https://github.com/microsoft/terminal/pull/10638#discussion_r674151375