Remove test_utils::RecursiveUnlinkDir().

This test-only function has an equivalent implementation in libcrhome
so we remove it from update_engine and use the library version instead.

Bug: None
Test: FEATURES=test emerge-link update_engine

Change-Id: I3027c879d40461dbdf8b52dd1b65da0240549c9c
diff --git a/update_attempter_unittest.cc b/update_attempter_unittest.cc
index 31f75b6..c2318f0 100644
--- a/update_attempter_unittest.cc
+++ b/update_attempter_unittest.cc
@@ -163,7 +163,7 @@
   }
 
   void TearDown() override {
-    test_utils::RecursiveUnlinkDir(test_dir_);
+    base::DeleteFile(base::FilePath(test_dir_), true);
   }
 
  public: