Removed unused imports

This fixes compilation errors in 4.15 M3

Bug: 560210
Change-Id: I7f36edc7fb83c66c8889d7573b4439d5a52334e4
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/dfs/DfsGarbageCollectorTest.java

@@ -19,7 +19,6 @@ import java.util.Collections;
 import java.util.concurrent.TimeUnit;
 
 import org.eclipse.jgit.internal.storage.dfs.DfsObjDatabase.PackSource;
-import org.eclipse.jgit.internal.storage.dfs.DfsRefDatabase;
 import org.eclipse.jgit.internal.storage.reftable.RefCursor;
 import org.eclipse.jgit.internal.storage.reftable.ReftableConfig;
 import org.eclipse.jgit.internal.storage.reftable.ReftableReader;

org.eclipse.jgit.test/tst/org/eclipse/jgit/util/StatsTest.java

@@ -12,7 +12,6 @@ package org.eclipse.jgit.util;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
 
-import org.eclipse.jgit.util.Stats;
 import org.junit.Test;
 
 public class StatsTest {