bitbake: hob: remove custom image from the images list when not needed

bitbake: hob: remove custom image from the images list when not needed

Selected custom image recipes should be cleared from the combo the moment
you change your selection. The idea is to always perform the selection of
those images in the same way (i.e through the "Select from my image
recipes" option).

[YOCTO #5001]
(Bitbake rev: 94483ee5ae9f4051bccd660c4718c36564e17161)

Signed-off-by: Cristiana Voicu <cristiana.voicu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>