tty: vt: remove unused consw::con_flush_scrollback()

consw::con_flush_scrollback() is unused since commit 973c096f
(vgacon: remove software scrollback support). Drop it.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Tested-by: Helge Deller <deller@gmx.de> # parisc STI console
Link: https://lore.kernel.org/r/20240122110401.7289-45-jirislaby@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>