Use native constructors instead of Guava to instantiate empty collections

It's not necessary to use Guava's helper methods when instantiating
empty collections. Just use the native constructors.

Change-Id: I720b46400b4548a21593b97ca48055e671272427

src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java b/src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java
index 04fe7d6..5274df9 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java
@@ -19,7 +19,6 @@
 import com.google.common.base.Strings;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Lists;
-import com.google.common.collect.Sets;
 import com.google.gerrit.common.Nullable;
 import com.google.gerrit.common.data.GroupDescription;
 import com.google.gerrit.common.data.GroupReference;
@@ -48,6 +47,7 @@
 
 import java.util.Collection;
 import java.util.Collections;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
 
@@ -158,7 +158,7 @@
     }
     try {
       AccountControl ctl = accountControlFactory.get();
-      Set<Account.Id> ids = Sets.newHashSet();
+      Set<Account.Id> ids = new HashSet<>();
       List<GroupReference> matches = Lists.newArrayListWithCapacity(MAX);
       String a = name;
       String b = end(a);