tty/hvc: remove celleb-only beat driver

The beat hvc driver is only used by celleb.
celleb has been dropped [1], so drop the drivers.

[1] http://patchwork.ozlabs.org/patch/451730/

CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
CC: Jiri Slaby <jslaby@suse.cz>
CC: Valentin Rothberg <valentinrothberg@gmail.com>
CC: mpe@ellerman.id.au
CC: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>