ArmPlatformPkg/Bds: Fix delete boot option

- We need to delete the boot option variable from storage not just
  adjust the BootOrder variable.
- The Linux tool 'efibootmgr' still showed the previously removed boot
  options.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Harry Liebel <Harry.Liebel@arm.com>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16070 6f19259b-4bc3-4df7-8a09-765794883524