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/utils_unittest.cc b/utils_unittest.cc
index a315e53..2bdb76b 100644
--- a/utils_unittest.cc
+++ b/utils_unittest.cc
@@ -103,7 +103,7 @@
   EXPECT_FALSE(utils::IsSymlink(temp_file.c_str()));
   EXPECT_TRUE(utils::IsSymlink(temp_symlink.c_str()));
   EXPECT_FALSE(utils::IsSymlink("/non/existent/path"));
-  EXPECT_TRUE(test_utils::RecursiveUnlinkDir(temp_dir));
+  EXPECT_TRUE(base::DeleteFile(base::FilePath(temp_dir), true));
 }
 
 TEST(UtilsTest, SplitPartitionNameTest) {