scsi: core: Remove duplicate declarations

struct request and struct request_queue are declared twice.  Remove the
duplicate declarations.

Link: https://lore.kernel.org/r/20210327030850.918018-1-wanjiabing@vivo.com


Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>