POM cleanup

POM cleanup

- derived l2switch-parent from binding-parent to paick up yangtools
  definitions.

- removed yangtools.version and other properties no longer needed in
  l2switch-parent pom.

- removed unnecesrary yang-maven-plugin definitions.

Change-Id: I0a0b07f72e5a53fc09f411d32af8af6ceeade6a4
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>