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).