drop custom scmpublish lifecycle with its scmpublish goal

when preparing a new release, generating the site with Java 8, I found that the aggregate tag for the scmpublish goal contained a typo: it was written @aggregate when it should have been @aggregator (then makes Javadoc 8 fail)
this proves that this feature has never really been used, and can be dropped: this will simplify the code

(notice that in addition, it was not possible to switch from plugin-tools javadoc annotation to Java annotations, because Java annotations don't permit custom lifecycle)