MdeModulePkg RamDiskDxe: Do not save 'Size' numeric value by varstore

MdeModulePkg RamDiskDxe: Do not save 'Size' numeric value by varstore

The 'Size' numeric value used when creating a raw RAM disk does not
require a varstore to save its previous value in the create raw RAM disk
HII page.

The expecting behavior is that after a user created a raw RAM disk, the
next time when the create raw RAM disk page is entered, the 'Size' numeric
will be the default value (EFI_PAGE_SIZE).

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Tapan Shah <tapandshah@hpe.com>