[SCSI] mpt2sas: Remove unused duplicate diag_buffer_enable param

Commit 921cd802 ("[SCSI] mpt2sas: New feature - Fast Load
Support") moved handling of the diag_buffer_enable module parameter
from mpt2sas_base.c to mpt2sas_scsih.c, but it left an old copy of the
parameter in mpt2sas_base.c.  Remove the unused stub.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Acked-by: "Nandigama, Nagalakshmi" <Nagalakshmi.Nandigama@lsi.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>