Disable chgrp event logs

The creation of `CHGRP` event logs for the planeinfo elements of screens (or even just ` a&exposureTime=1.0&sizeT=10000.fake` with openmicroscopy/bioformats#795) was taking significant time. In fact the `onRelease` method was taking nearly half of the time just to publish events. Since CHGRP events do not take part in indexing (as do the DELETE event logs), this optimization should be generally safe.

To test:
 * touch and import a fake file of the form above and import (**note:** this generates a +2GB pixels file under OMERO4)
 * run `bin/omero chgrp A /Image:...` on the newly created image.

Without this PR, that move will take many minutes, if it does not OOM the server. With it, it should take less than a minute (e.g. on my fairly fast linux box, ~20 seconds, as opposed to the 5+ minutes I waited before Ctrl-C'ing it without the PR )

/cc @jburel, @pwalczysko, @chris-allan