distro: remove ptest from DISTRO_FEATURES in production builds

Test and debug images use ptest for runtime testing. However, it is not
used in production builds, and should not be. They add to the build time.

From our results, a build of oniro-image-base with ptest takes 160min,
while without it is only 110 min.

oniro-image-extra-test was the only test image that wasn't adding this
DISTRO_FEATURE explicitly.

Signed-off-by: Marta Rybczynska <marta.rybczynska@linaro.org>