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/delta_performer_unittest.cc b/delta_performer_unittest.cc
index b177790..db19729 100644
--- a/delta_performer_unittest.cc
+++ b/delta_performer_unittest.cc
@@ -673,7 +673,7 @@
   install_plan_.public_key_rsa = "not-valid-base64";
   EXPECT_FALSE(performer_.GetPublicKeyFromResponse(&key_path));
 
-  EXPECT_TRUE(test_utils::RecursiveUnlinkDir(temp_dir));
+  EXPECT_TRUE(base::DeleteFile(base::FilePath(temp_dir), true));
 }
 
 TEST_F(DeltaPerformerTest, ConfVersionsMatch) {