docs: fb: Remove vesafb scrollback boot option

In commit 50145474f6 ("fbcon: remove soft scrollback code"), the
framebuffer scrollback mode was removed, but the documentation was not
updated.  Properly update the documentation by removing the option that
is no longer present,remove the scrollback option.

Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/799e4d6ecca9aab53e823fc0001887d728be0aa5.1600333774.git.unixbhaskar@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/fb/vesafb.rst

@@ -135,8 +135,6 @@ ypan	  enable display panning using the VESA protected mode
 
                 * scrolling (fullscreen) is fast, because there is
 		  no need to copy around data.
-		* You'll get scrollback (the Shift-PgUp thing),
-		  the video memory can be used as scrollback buffer
 
           kontra: