mt_ventoux: remove board

This board still doesn't select CONFIG_DM and seems to be umaintained.
As it makes progress on modernizing several DaVinci drivers more
difficult and the maintainer has not expressed interest in updating
it, this patch proposes to remove it.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Stefano Babic <sbabic@denx.de>