tty: vt: drop checks for undefined VT_SINGLE_DRIVER

VT_SINGLE_DRIVER is defined nowhere. Remove its checks. These were added
long time ago and never used.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230420093559.13200-4-jirislaby@kernel.org


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