Aggregate json annotations in uber-jar

[JES: attached the exec:java phase to the lifecycle to make sure that
the annotation combiner is run; also changed the package to avoid the
deprecated annotation combiner class; while at it, avoid writing to
src/assembly/all/ and instead write to target/assembly/all/]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Remove obsolete annotation aggregation readme