Reverts flutter/flutter#144207

Initiated by: CaseyHillers

Reason for reverting: b/327301206 - Breaking a customer test

Original PR Author: LongCatIsLooong

Reviewed By: {gspencergoog}

This change reverts the following previous change:
Original Description:
`FocusNode.canRequestFocus` was doing a double traversal if no ancestor disallows focus. The last for loop only has to reach as far as the enclosing scope.

Also this caches the `FocusNode.enclosingScope` since the getter access happens much more frequently than node reparenting.