Fix duplicate presubmit entries in thread/TEST_MAPPING

The duplicate entries make `CtsThreadNetworkTestCases` not included in presubmit. The `CtsThreadNetworkTestCases` was added in aosp/2804113 but was accidentally removed in aosp/2888526.

Bug: 317769267
Bug: 297729075

Test: presubmit

Change-Id: I184d21deddc0f0c247321a8d1afe5a8f8674f974
diff --git a/thread/TEST_MAPPING b/thread/TEST_MAPPING
index d3765f1..6a5ea4b 100644
--- a/thread/TEST_MAPPING
+++ b/thread/TEST_MAPPING
@@ -2,9 +2,7 @@
   "presubmit": [
     {
       "name": "CtsThreadNetworkTestCases"
-    }
-  ],
-  "presubmit": [
+    },
     {
       "name": "ThreadNetworkUnitTests"
     }