efi_loader: simplify efi_disk_remove

Instead of discovering the ID of the device and call two different
functions for a block device or a partition, we can rewrite
efi_disk_remove() and handle the minor differences between the two
variants internally.  As a results we can simplify efi_disk_remove()
a lot and get rid of the extra efi_disk_delete_raw/blk calls.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

If a handle is not found, return 0 to let the device be removed.

Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>