Remove unused function

I believe this did not work in the first place. CompletableFuture#get()
might throw a timeout exception if TIMEOUT_MS is reached which fails the
assertion (but it should not!).

Anyway, it is unused, so just delete this method.

Test: TH
Change-Id: I43a747179eeaded320df32f40f1237be11de7867
diff --git a/tests/cts/net/src/android/net/cts/EthernetManagerTest.kt b/tests/cts/net/src/android/net/cts/EthernetManagerTest.kt
index 6fa2812..61ebd8f 100644
--- a/tests/cts/net/src/android/net/cts/EthernetManagerTest.kt
+++ b/tests/cts/net/src/android/net/cts/EthernetManagerTest.kt
@@ -303,18 +303,6 @@
         fun expectOnAvailable(timeout: Long = TIMEOUT_MS): String {
             return available.get(timeout, TimeUnit.MILLISECONDS)
         }
-
-        fun expectOnUnavailable() {
-            // Assert that the future fails with the IllegalStateException from the
-            // completeExceptionally() call inside onUnavailable.
-            assertFailsWith(IllegalStateException::class) {
-                try {
-                    available.get(TIMEOUT_MS, TimeUnit.MILLISECONDS)
-                } catch (e: ExecutionException) {
-                    throw e.cause!!
-                }
-            }
-        }
     }
 
     private class EthernetOutcomeReceiver :