[hotfix][network] Simplify SingleInputGate#isFinished

Previous implementation of #isFinished was not only checking whether the input gate is finished, but also if it was
closed. This commit simplifies #isFinished to just check whether the input gate is finished. The only difference is that
the following scenario:

inputGate.close()
inputGate.isFinished() (2)

before this change (2) would return true, now it will return false. This potentially could be
relevant, because TaskCanceller is calling InputGate#close, however it doesn't relay on #isFinished().

Any InputGate's user after accessing InputGate#get/pollNextBufferOrEvent would still get an
IllegalStateException("Released") both before and after this change, so changing/simplifying #isFinished()
implementation from "isFinishedOrClosed()" as it was before to just "isFinished()" shouldn't matter.

flink-runtime/src/main/java/org/apache/flink/runtime/io/network/partition/consumer/SingleInputGate.java

@@ -447,15 +447,7 @@ public void close() throws IOException {
 
  @Override
  public boolean isFinished() {
- synchronized (requestLock) {
- for (InputChannel inputChannel : inputChannels.values()) {
- if (!inputChannel.isReleased()) {
- return false;
- }
- }
- }
-
- return true;
+ return hasReceivedAllEndOfPartitionEvents;
  }
 
  @Override