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

Documentation/kernel-parameters.txt

@@ -1619,8 +1619,6 @@ and is between 256 and 4096 characters. It is defined in the file
 
 	serialnumber	[BUGS=X86-32]
 
-	sg_def_reserved_size=	[SCSI]
-
 	shapers=	[NET]
 			Maximal number of shapers.