POM: remove obsolete comment
POM: use ${main-class} for pom-scijava goodness

The pom-scijava project now provides a generic exec profile using
${main-class}, which we can now use instead of ${imagej.main-class}.

As such, this project no longer needs its own exec profile. A
similar profile for jdb was also removed, which will appear in
pom-scijava next release:

https://github.com/scijava/pom-scijava/commit/3fb0372028ec0215ccbdb0720e26f28be8066030
POM: remove obsolete javadoc profile

It moved to its own project:
  https://github.com/imagej/imagej-javadoc
POM: fix typo in comment
POM: update URL