Drop yangtools-parent

Drop yangtools-parent

This emerged from trying to use odlparent's bundle-parent for bundles,
and thus be able to remove the yangtools overrides for
maven-jar-plugin and maven-javadoc-plugin in particular.

This probably removes features from yangtools-parent which should be
replicated elsewhere; in particular, the Sonar/JaCoCo integration
should be available for everyone.

Change-Id: Id41e61eb3da2f30be29981a11387bde467acae3d
Signed-off-by: Stephen Kitt <skitt@redhat.com>