install-imagej: lean on the gen-scripts script

This script will be (hopefully) distributed as part of the OMERO-5.0
update site, so will be present after ImageJ is bootstrapped. Let's
just use it directly, instead of replicating any logic in this script.
gen-scripts: add TODO reminder
Remove obsolete launcher scripts

We definitely do not want to be wrapping the "jython" executable with
our own hacky shell script. And the "imagej" launcher is no longer
necessary now that we are distributing all required OMERO JARs with the
OMERO-5.0 update site, and no longer need to add JARs from the actual
OMERO installation's lib/server directory to the classpath.
install-imagej: fix references to installer script
gen-scripts: always use $0/.. for ImageJ directory

This is because the gen-scripts script is expected to be distributed via
the OMERO update sites, and thus live in the lib directory of the ImageJ
installation.
WIP: how to make the installation better