docs: fb: Remove matroxfb scrollback boot option

In commit 50145474 ("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 pressent, and removed the particular scrollback option.

Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/e5187e70faf8237825a142313802983a6c214baa.1600333774.git.unixbhaskar@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>