docs: fb: Remove sstfb scrollback boot option

In commit 50145474f6ef ("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 i.e remove a stanza related to scrollback.

Signed-off-by: Bhaskar Chowdhury <[e-mail]>
Reviewed-by: Greg Kroah-Hartman <[e-mail]>
Link: https://lore.kernel.org/r/d7e[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/fb/sstfb.rst

···
   contact me.
 - The 24/32 is not likely to work anytime soon, knowing that the
   hardware does ... unusual things in 24/32 bpp.
-- When used with another video board, current limitations of the linux
-  console subsystem can cause some troubles, specifically, you should
-  disable software scrollback, as it can oops badly ...
 
 Todo
 ====