gw_ventana: Migrate to using CONFIG_EXTRA_ENV_TEXT

Move the environment text over from being set via
CONFIG_EXTRA_ENV_SETTINGS in include/configs/gw_ventana.h and over
to plain text in board/gateworks/gw_ventana/gw_ventana.env.  This lets
us drop CONFIG_EXTRA_ENV_SETTINGS_COMMON as everything resides in a
single environment file now.

Cc: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Tim Harvey <tharvey@gateworks.com>