Partial revert of hosts/regions work to fix NPE with null provider.

Change-Id: I7dacad818ca9ffa2ea8085c499990c8e92644607
diff --git a/core/api/src/main/java/org/onosproject/net/region/RegionStore.java b/core/api/src/main/java/org/onosproject/net/region/RegionStore.java
index a21a1f9..49cc378 100644
--- a/core/api/src/main/java/org/onosproject/net/region/RegionStore.java
+++ b/core/api/src/main/java/org/onosproject/net/region/RegionStore.java
@@ -17,7 +17,6 @@
 
 import org.onosproject.cluster.NodeId;
 import org.onosproject.net.DeviceId;
-import org.onosproject.net.HostId;
 import org.onosproject.store.Store;
 
 import java.util.Collection;
@@ -111,12 +110,4 @@
      */
     void removeDevices(RegionId regionId, Collection<DeviceId> deviceIds);
 
-    /**
-     * Returns the set of hosts that belong to the specified region.
-     *
-     * @param regionId region identifier
-     * @return set of identifiers for hosts in the given region
-     */
-    Set<HostId> getRegionHosts(RegionId regionId);
-
 }
diff --git a/core/net/src/main/java/org/onosproject/net/region/impl/RegionManager.java b/core/net/src/main/java/org/onosproject/net/region/impl/RegionManager.java
index 75d96ed..c7b8342 100644
--- a/core/net/src/main/java/org/onosproject/net/region/impl/RegionManager.java
+++ b/core/net/src/main/java/org/onosproject/net/region/impl/RegionManager.java
@@ -16,6 +16,7 @@
 
 package org.onosproject.net.region.impl;
 
+import com.google.common.collect.ImmutableSet;
 import org.apache.felix.scr.annotations.Activate;
 import org.apache.felix.scr.annotations.Component;
 import org.apache.felix.scr.annotations.Deactivate;
@@ -154,7 +155,8 @@
     public Set<HostId> getRegionHosts(RegionId regionId) {
         checkPermission(REGION_READ);
         checkNotNull(regionId, REGION_ID_NULL);
-        return store.getRegionHosts(regionId);
+        // TODO: compute hosts from region devices
+        return ImmutableSet.of();
     }
 
 }
diff --git a/core/store/dist/src/main/java/org/onosproject/store/region/impl/DistributedRegionStore.java b/core/store/dist/src/main/java/org/onosproject/store/region/impl/DistributedRegionStore.java
index e101514..6d70376 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/region/impl/DistributedRegionStore.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/region/impl/DistributedRegionStore.java
@@ -27,7 +27,6 @@
 import org.onlab.util.Identifier;
 import org.onosproject.cluster.NodeId;
 import org.onosproject.net.DeviceId;
-import org.onosproject.net.HostId;
 import org.onosproject.net.region.DefaultRegion;
 import org.onosproject.net.region.Region;
 import org.onosproject.net.region.RegionEvent;
@@ -79,7 +78,6 @@
 
     private ConsistentMap<RegionId, Set<DeviceId>> membershipRepo;
     private Map<RegionId, Set<DeviceId>> regionDevices;
-    private Map<RegionId, Set<HostId>> regionHosts;
 
     private Map<DeviceId, Region> regionsByDevice = new HashMap<>();
 
@@ -142,12 +140,6 @@
     }
 
     @Override
-    public Set<HostId> getRegionHosts(RegionId regionId) {
-        Set<HostId> hostIds = regionHosts.get(regionId);
-        return hostIds != null ? ImmutableSet.copyOf(hostIds) : ImmutableSet.of();
-    }
-
-    @Override
     public Region createRegion(RegionId regionId, String name, Region.Type type,
                                List<Set<NodeId>> masterNodeIds) {
         return regionsRepo.compute(regionId, (id, region) -> {