efi_loader: remove efi_disk_is_system_part()

The block IO protocol may be installed on any handle. We should make
no assumption about the structure the handle points to.

efi_disk_is_system_part() makes an illegal widening cast from a handle
to a struct efi_disk_obj. Remove the function.

Fixes: Fixes: 41fd5068 ("efi_loader: disk: add efi_disk_is_system_part()")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>