Remove checkstyle plugin declaration
Remove checkstyle plugin declaration

Checkstyle should not be run in aggregator poms. This patch fixes
a maven warning about missing version and should result in faster
compilation times.

Change-Id: I07d810fd05568865eb31f2f00fcc482fa4d0e31f
Signed-off-by: Robert Varga <91ae5aa8c7a9f7e57b701db766e23e544aaa6ae9@pantheon.tech>

opendaylight/yangtools,opendaylight/yangtools