yocto/domu-image: Remove testimage from IMAGE_CLASSES

When the rm_work bbclass is used to reduce disk usage, an unwanted
dependency is inserted from domuX-image:do_rm_work on
domuX-image:do_testimage.

To prevent this, explicitly remove testimage from the DomU image
recipes.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>