[REFACTOR] Delete ActivityObjects

Since all fields of all Activities can now be handled by XStream, there
is no need for ActivityDataObjects -- which were an easier-to-serialize
version of Activities -- anymore.
All XStream annotation were moved from the ActivityDataObject class to
the corresponding Activity class, so the resulting XML serialization
should be unchanged.

Change-Id: If98e779cee6f28fd7a0ff63f8654297916250d3a
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1526
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>