scsi: docs: Remove obsolete scsi typedef text from scsi_mid_low_api

Commit 91ebc1fa ("scsi: core: remove Scsi_Cmnd typedef") removed the
Scsi_cmnd typedef but it was still mentioned in a paragraph in the "SCSI
mid_level - lower_level driver interface" documentation page.  Remove this
obsolete paragraph.

Link: https://lore.kernel.org/r/20200905210211.2286172-1-nfraprado@protonmail.com


Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Suggested-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@protonmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>