Re-enable test that was causing TimeoutExceptions on ARM64.

The libcore test libcore.java.lang.OldSystemTest#test_gc has
been failing with a java.util.concurrent.TimeoutException on
ART Builbot's ARM64 concurrent collector configuration.  No
longer ignore this failure to see whether recent changes
(namely https://android-review.googlesource.com/#/c/204125)
fixed the issue.

Bug: 26155567
Change-Id: I5cd3c3956cfd510bab1cb9d3b9479560f9703e6d
diff --git a/tools/libcore_failures_concurrent_collector.txt b/tools/libcore_failures_concurrent_collector.txt
index 75d1eff..95f0c2d 100644
--- a/tools/libcore_failures_concurrent_collector.txt
+++ b/tools/libcore_failures_concurrent_collector.txt
@@ -16,11 +16,5 @@
   names: ["jsr166.LinkedTransferQueueTest#testTransfer2",
           "jsr166.LinkedTransferQueueTest#testWaitingConsumer"],
   bug: 25883050
-},
-{
-  description: "libcore.java.lang.OldSystemTest#test_gc failure on armv8-concurrent-collector.",
-  result: EXEC_FAILED,
-  names: ["libcore.java.lang.OldSystemTest#test_gc"],
-  bug: 26155567
 }
 ]