EthernetTetheringTest: remove isEthernetTetheringSupported

isEthernetTetheringSupported() has been not used since
using TetheringManager#isTetheringSupported in aosp/2143272.

Bug: 258637850
Test: EthernetTetheringTest
Change-Id: Ifc560613301393eeadb6df11688944b2dc629c35
diff --git a/Tethering/tests/integration/src/android/net/EthernetTetheringTest.java b/Tethering/tests/integration/src/android/net/EthernetTetheringTest.java
index f0f9a31..c2c9fc4 100644
--- a/Tethering/tests/integration/src/android/net/EthernetTetheringTest.java
+++ b/Tethering/tests/integration/src/android/net/EthernetTetheringTest.java
@@ -556,23 +556,6 @@
         // client, which is not possible in this test.
     }
 
-    private boolean isEthernetTetheringSupported() throws Exception {
-        final CompletableFuture<Boolean> future = new CompletableFuture<>();
-        final TetheringEventCallback callback = new TetheringEventCallback() {
-            @Override
-            public void onSupportedTetheringTypes(Set<Integer> supportedTypes) {
-                future.complete(supportedTypes.contains(TETHERING_ETHERNET));
-            }
-        };
-
-        try {
-            mTm.registerTetheringEventCallback(mHandler::post, callback);
-            return future.get(TIMEOUT_MS, TimeUnit.MILLISECONDS);
-        } finally {
-            mTm.unregisterTetheringEventCallback(callback);
-        }
-    }
-
     private static final class MyTetheringEventCallback implements TetheringEventCallback {
         private final TetheringManager mTm;
         private final CountDownLatch mTetheringStartedLatch = new CountDownLatch(1);