Make the removal of proxies work in the Synchronization Client

When the synchronization client tries to remove deleted proxies, it fails because it tries to delete the version first and the version cannot be deleted since proxies are pointing to it.

The log is:

java.lang.IllegalStateException: Row already registered:
da336f6d-0413-4444-8484-5586bbf75806
at org.nuxeo.ecm.core.storage.sql.Context.create(Context.java:192) at
org.nuxeo.ecm.core.storage.sql.HierarchyContext.create(HierarchyContext.java:145) at
org.nuxeo.ecm.core.storage.sql.PersistenceContext.createSimpleFragment(PersistenceContext.java:355) at
org.nuxeo.ecm.core.storage.sql.SessionImpl.addChildNode(SessionImpl.java:420) at
org.nuxeo.ecm.core.storage.sql.coremodel.SQLSession.importChild(SQLSession.java:618) at
org.nuxeo.ecm.core.storage.sql.coremodel.SQLSession.importDocument(SQLSession.java:384) at
org.nuxeo.ecm.core.api.AbstractSession.importDocument(AbstractSession.java:834) at
org.nuxeo.ecm.core.api.AbstractSession.importDocuments(AbstractSession.java:802) at
org.nuxeo.ecm.platform.sync.processor.TupleProcessorAdd$UnrestrictedImport.run(TupleProcessorAdd.java:69) at
org.nuxeo.ecm.core.api.UnrestrictedSessionRunner.runUnrestricted(UnrestrictedSessionRunner.java:87) at
org.nuxeo.ecm.platform.sync.processor.TupleProcessorAdd.runUnrestrictedImport(TupleProcessorAdd.java:49) at
org.nuxeo.ecm.platform.sync.processor.TupleProcessorAddVersion.process(TupleProcessorAddVersion.java:75) at
org.nuxeo.ecm.platform.sync.manager.DocumentsSynchronizeManager.addDocuments(DocumentsSynchronizeManager.java:129) at
org.nuxeo.ecm.platform.sync.manager.DocumentsSynchronizeManager.run(DocumentsSynchronizeManager.java:109) at
org.nuxeo.ecm.platform.sync.client.SynchronizeServiceImpl.doSynchronizeDocuments(SynchronizeServiceImpl.java:72) at
org.nuxeo.ecm.platform.sync.client.SynchronizeServiceImpl.doSynchronize(SynchronizeServiceImpl.java:91) at
org.nuxeo.rcp.embedded.sync.SynchronizationJob.synchronize(SynchronizationJob.java:77) at
org.nuxeo.rcp.embedded.sync.SynchronizationJob.run(SynchronizationJob.java:100) at org.eclipse.core.internal.jobs.Worker.run(Worker.java:55)