dt-bindings: display: remove zte,vou.txt binding doc

The zte zx platform was removed in commit 89d4f98 ("ARM: remove zte
zx platform") and the zxdrm driver is going to be removed in v5.15 via
drm tree. Let's remove the now obsolete binding doc.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jun Nie <jun.nie@linaro.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20210831034924.86-1-yuzenghui@huawei.com
Signed-off-by: Rob Herring <robh@kernel.org>