OioSctpChannel iterating over selected keys

Motivation:
OioSctpChannel.doReadMessages is iterating over the selected keys, and ignoring each selected key. It is not known why this is needed and no other channel implementation does this.

Modifications:
- Stop iterating over selected keys, and just read like other channels

Result:
No unnecessary iteration in OioSctpChannel.doReadMessages.
Fixes https://github.com/netty/netty/issues/3884

transport-sctp/src/main/java/io/netty/channel/sctp/oio/OioSctpChannel.java

@@ -183,9 +183,6 @@ public class OioSctpChannel extends AbstractOioMessageChannel
             return readMessages;
         }
 
-        Set<SelectionKey> reableKeys = readSelector.selectedKeys();
-        try {
-            for (@SuppressWarnings("unused") SelectionKey ignored : reableKeys) {
         final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
         ByteBuf buffer = allocHandle.allocate(config().getAllocator());
         boolean free = true;
@@ -210,10 +207,6 @@ public class OioSctpChannel extends AbstractOioMessageChannel
                 buffer.release();
             }
         }
-            }
-        } finally {
-            reableKeys.clear();
-        }
         return readMessages;
     }