MIPS: JZ4740: Qi LB60: Remove support for AVT2 variant

AVT2 was a prototype board of which about 5 were made, none of which
are in use anymore.

Signed-off-by: Maarten ter Huurne <maarten@treewalker.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Paul Cercueil <paul@crapouillou.net>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13103/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>