esp: revert 75ef849696 "esp: correctly fill bus id with requested lun"

esp: revert 75ef849696 "esp: correctly fill bus id with requested lun"

This commit from nearly 10 years ago is now broken due to the improvements
in esp emulation (or perhaps was never correct).  It shows up as a bug
in detecting the CDROM drive under MacOS. The error is caused by the
MacOS CDROM driver sending this CDB with an "S without ATN" command and
without DMA:

    0x12 0x00 0x00 0x00 0x05 0x00 (INQUIRY)

This is a valid INQUIRY command, however with this logic present the 3rd
byte (0x0) is copied over the 1st byte (0x12) which silently converts the
INQUIRY command to a TEST UNIT READY command before passing it to the
QEMU SCSI layer.  Since the TEST UNIT READY command has a zero length
response the MacOS CDROM driver never receives a response and assumes
the CDROM is not present.

The logic was to ignore the IDENTIFY byte and copy the LUN over from
the CDB, which did store the LUN in bits 5-7 of the second byte in
olden times.  This however is all obsolete, so just drop the code.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20210519100803.10293-5-mark.cave-ayland@ilande.co.uk>
[Tweaked commit message. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>