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