Deactivate xtend-maven-plugin

We have only three artifacts which use xtend, so there is no reason to
mention it in the build section. Move the configuration into
pluginManagement, speeding up the build.

Change-Id: Iac5e6e93a723b5f7e8beeb7dfa0d63d316d2f6e0
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Deactivate xtend-maven-plugin"