Relax CI depending on Nuxeo master on sandbox projects

The template for sandbox projects at https://qa.nuxeo.org/jenkins/job/Sandbox/job/template_sandbox/ seems to be embedding a downstream job dependency to the addons-master job at https://qa.nuxeo.org/jenkins/job/master/job/addons-master/

That means every nuxeo-master job will trigger all sandbox addons build too, which is unnecessary in some cases.

Random samples taken from the ~ 60 jobs that are currently downstreams of addons-master (using 10.3-SNAPSHOT):

• https://qa.nuxeo.org/jenkins/job/Sandbox/job/sandbox_nuxeo-zip-utils-master aligned on 10.2
• https://qa.nuxeo.org/jenkins/job/Sandbox/job/sandbox_nuxeo-webpage-archiver-master aligned on 9.10-SNAPSHOT
• https://qa.nuxeo.org/jenkins/job/Sandbox/job/sandbox_nuxeo-dam-dashboard-parent-master aligned on 8.10
• https://qa.nuxeo.org/jenkins/job/Sandbox/job/sandbox_nuxeo-labs-master aligned on 10.1

It seems the sandbox projects are not really following the step of Nuxeo releases, hence there's not need for implicit CI on their master branch.

That removal should lower the load on Jenkins when there are frequent commits on the nuxeo platform code.

It should still be possible to setup that dependency from the project configuration, like it's done on other templates, if the sandbox project is indeed aligned on the nuxeo master branch version.