Documentation: Delete final reference to "sg_def_reserved_size" kernel parm

given the following:

$ grep -rw sg_def_reserved_size *
Documentation/kernel-parameters.txt:    sg_def_reserved_size=   [SCSI]
$

that kernel parameter looks exceedingly dead.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>