Drop imx6_CI_image_build and imx8_CI_image_build
@angus.ainslie We have
https://arm01.puri.sm/job/Images/job/imx6_CI_image_build/
and
https://arm01.puri.sm/job/Images/job/imx8_CI_image_build/
which do very similar things than the "Image Build" jobs, can these be removed?