Eliminate usages of net.imagej.ImageJ

This change is in preparation for reducing the dependencies of the
imagej-omero component. While net.imagej:imagej is very convenient as a
starting point and for casual developers, for robust library components
like imagej-omero it makes more sense to do the dependencies properly,
declaring only those dependencies which are actually needed.
Remove superfluous Main class

It was useful for testing, but dragged
in a dependency on net.imagej:imagej.
POM: remove no-longer-used net.imagej:imagej dep

This reduces the dependency graph of imagej-omero by half!
(from 106 to 56 artifacts)
Merge branch 'reduce-deps'

This eliminates the dependency on the toplevel net.imagej:imagej
artifact, which cuts the dependency footprint by ~50%.