Switch to more appropriate parent

In pom-imagej, we have pom-scijava as implicit parent, still.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
pom-imagej already manages the ImageJ 1.x dependency

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
pom-imagej is served from Maven Central

... therefore, we do not need to add the ImageJ Maven repository
section anymore.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>