Revert "Blacklist flaky test."

Reverts the blacklisting of NetworkSecurityPolicyTest
#testCleartextTrafficPolicyWithJarHttpURLConnection.

This reverts commit 71ceb721240d50d396ea1a61a2bf2832be345007.

Bug: 31944681
Change-Id: Ib0f13cfec021e258ceba480c43cced59c554b3f5
diff --git a/tools/libcore_failures.txt b/tools/libcore_failures.txt
index 872ec13..a69b58b 100644
--- a/tools/libcore_failures.txt
+++ b/tools/libcore_failures.txt
@@ -234,11 +234,5 @@
   modes: [device],
   names: ["libcore.java.lang.ProcessBuilderTest#testRedirectInherit",
           "libcore.java.lang.ProcessBuilderTest#testRedirect_nullStreams"]
-},
-{
-  description: "Sometimes timeouts",
-  result: EXEC_FAILED,
-  bug: 31258002,
-  names: ["libcore.net.NetworkSecurityPolicyTest#testCleartextTrafficPolicyWithJarHttpURLConnection"]
 }
 ]