Prepare pom.xml for future deployments to Maven Central

We actually deployed ImageJ 1.48c to Maven Central already, under the
GAV net.imagej:ij:1.48c, using the change introduced by this commit: we
now avoid calling javadoc with the -use flag (there is a long-time bug
in Java 6 that prevents -use from working correctly in case that there
are classes in the default package -- and ImageJ 1.x *does* have such
classes in its source code).

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