tests: Remove incorrect bitbake variable.

This variable is actually used to detect that we are building with
Yocto. With the mender-convert rewrite, the relevant section started
working in mender-convert as well, so remove this, and also remove the
check in mender-image-tests.

Changelog: None

Signed-off-by: Kristian Amlie <kristian.amlie@northern.tech>

mender-convert-package

@@ -307,5 +307,4 @@ EOF
 cat <<- EOF >> deploy/${image_name}.cfg
 IMAGE_FSTYPES="${image_fs_type} mender sdimg"
 ARTIFACTIMG_FSTYPE="${image_fs_type}"
-LAYER_CONF_VERSION="2"
 EOF