Do not check the return value of Future.cancel()

Future.cancel() is expected to return false if the future is done or had been cancelled
previously, and there doesn't seem to be a way to check whether or not that is the case
without a race condition.

RELNOTES: None.
PiperOrigin-RevId: 339686101

src/main/java/com/google/devtools/build/lib/dynamic/DynamicSpawnStrategy.java

@@ -138,8 +138,7 @@ private static void stopBranch(
       // reference to its own identifier wins and is allowed to issue the cancellation; the other
       // branch just has to give up execution.
       if (strategyThatCancelled.compareAndSet(null, cancellingStrategy)) {
-        boolean cancelled = branch.cancel(true);
-        checkState(cancelled, "Failed to cancel other branch from %s", cancellingStrategy);
+        branch.cancel(true);
         branchDone.acquire();
       } else {
         throw new DynamicInterruptedException(