Enable and fix Eclipse warnings about redundant type arguments

Since Java 7 it is possible to replace the type arguments required to
invoke the constructor of a generic class with an empty set of type
parameters (<>) as long as the compiler can infer the type arguments
from the context [1].

Enable the Eclipse warning about redundant type arguments, and remove
the ones it warns about.

[1] http://goo.gl/SG21kM

Change-Id: I422882949a6a6a57391580d881f73120b2843a0e

gerrit-server/src/main/java/com/google/gerrit/server/change/HashtagsUtil.java

@@ -135,6 +135,6 @@ public class HashtagsUtil {
             dbProvider.get());
       }
     }
-    return new TreeSet<String>(updatedHashtags);
+    return new TreeSet<>(updatedHashtags);
   }
 }