COMP: Improve speed of github azure CI build by dropping ITK4
COMP: Improve speed of github azure CI build by dropping ITK4

It was agreed at today's Elastix/SuperElastix sprint that ITK4 does not need to be supported anymore.

(The aim is now to allow elastix to build with ITK5 `ITK_LEGACY_REMOVE=ON`.)

SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix