efi_loader: remove unused members from struct efi_disk_obj

part and dev_index in struct efi_disk_obj are not used,
let's remove it.

This commit also removes the invalid structure comment
for @dev, it does not exist.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>