Bump Guava to 21.0

Change-Id: Id81705773d191db6a18153d8b2dd4abd2bfd0e60
diff --git a/core/common/src/main/java/org/onosproject/common/DefaultTopology.java b/core/common/src/main/java/org/onosproject/common/DefaultTopology.java
index dd2f3a9..81822c1 100644
--- a/core/common/src/main/java/org/onosproject/common/DefaultTopology.java
+++ b/core/common/src/main/java/org/onosproject/common/DefaultTopology.java
@@ -35,7 +35,6 @@
 import org.onlab.graph.TarjanGraphSearch;
 import org.onlab.graph.TarjanGraphSearch.SccResult;
 import org.onlab.graph.Weight;
-import org.onlab.util.GuavaCollectors;
 import org.onosproject.net.AbstractModel;
 import org.onosproject.net.ConnectPoint;
 import org.onosproject.net.DefaultDisjointPath;
@@ -437,7 +436,7 @@
         return KSHORTEST.search(graph, srcV, dstV, weigher, maxPaths)
                 .paths().stream()
                     .map(this::networkPath)
-                    .collect(GuavaCollectors.toImmutableSet());
+                    .collect(ImmutableSet.toImmutableSet());
     }
 
     /**