DefaultLegacyService: remove unused event handler

The event handler was never annotated with @EventHandler, so never
actually handled any events. And it didn't need to, since the UIService
was gleaned via the private uiService() helper method instead.
DefaultLegacyService: add missing @Deprecated tag
DefaultLegacyService: fix javadoc class references
DefaultLegacyService: remove unused import
Remove LegacyPluginFinder logic

We decided not to support legacy ImageJ 1.x commands directly from
modern ImageJ2 user interfaces. They _do_ appear in the ImageJ 1.x
legacy UI of course, but running them from any other UI results in a
host of compatibility problems.

We are _not_ removing the LegacyCommand, which an ImageJ2 command that
allows you to (limitedly) execute ImageJ 1.x commands. We just aren't
going to automatically discover and populate modern UIs with old-style
commands anymore, due to limitations and maintenance issues.