block: remove refs to XD disks from documentation

Commit d1a6f4f19728d6e90480e53601a90fc9f6a348ad
"block: delete super ancient PC-XT driver for 1980's hardware"
deleted the XD disk driver, but there are still a few
references to it in the documentation directory. Delete
the remnants and thus also free up the major block device
13 for reuse.

Cc: Paul Gortmaker <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

Documentation/devices.txt

···
 
 		Each device type has 5 bits (32 minors).
 
- 13 block	8-bit MFM/RLL/IDE controller
-		  0 = /dev/xda		First XT disk whole disk
-		 64 = /dev/xdb		Second XT disk whole disk
-
-		Partitions are handled in the same way as IDE disks
-		(see major number 3).
+ 13 block	Previously used for the XT disk (/dev/xdN)
+		Deleted in kernel v3.9.
 
  14 char	Open Sound System (OSS)
 		  0 = /dev/mixer	Mixer control

Documentation/kernel-parameters.txt

···
 			plus one apbt timer for broadcast timer.
 			x86_mrst_timer=apbt_only | lapic_and_apbt
 
-	xd=		[HW,XT] Original XT pre-IDE (RLL encoded) disks.
-	xd_geo=		See header of drivers/block/xd.c.
-
 	xen_emul_unplug=		[HW,X86,XEN]
 			Unplug Xen emulated devices
 			Format: [unplug0,][unplug1]

Documentation/m68k/kernel-options.txt

···
   /dev/sdd: -> 0x0830 (forth SCSI disk)
   /dev/sde: -> 0x0840 (fifth SCSI disk)
   /dev/fd : -> 0x0200 (floppy disk)
-  /dev/xda: -> 0x0c00 (first XT disk, unused in Linux/m68k)
-  /dev/xdb: -> 0x0c40 (second XT disk, unused in Linux/m68k)
 
   The name must be followed by a decimal number, that stands for the
 partition number. Internally, the value of the number is just