scsi: Drop scsi_req_abort

The only user of this function is spapr_vscsi.c. We can convert to
scsi_req_cancel plus adding a check in vscsi_request_cancelled.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
[Drop prototype. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>