Remove @deprecation keyword on AbstractUnsafe.ensureOpen(...)

Motivation:

e845670043 marked AbstractUnsafe.ensureOpen(...) as deprecated for no reason.

Modifications:

Remove `@deprecation`

Result:

Remove incorrect annotation

transport/src/main/java/io/netty/channel/AbstractChannel.java

@@ -880,7 +880,6 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
             return unsafeVoidPromise;
         }
 
-        @Deprecated
         protected final boolean ensureOpen(ChannelPromise promise) {
             if (isOpen()) {
                 return true;