[revert][misc] "modify check waitingForPingResponse with volatile (#1…

…2615)" (#19439)

This reverts commit 62e2547.

### Motivation

The motivation for #12615 relies on an incorrect understanding of Netty's threading model. The `ctx.executor()` is the context's event loop thread that is the same thread used to process messages. The `waitingForPingResponse` variable is only ever updated/read from the context's event loop, so there is no need to make the variable `volatile`.

### Modifications

* Remove `volatile` keyword for `waitingForPingResponse`

### Verifying this change

Read through all references to the variable.

### Documentation

- [x] `doc-not-needed`

### Matching PR in forked repository

PR in forked repository: Skipping for this trivial PR.

(cherry picked from commit fb28d83)

pulsar-common/src/main/java/org/apache/pulsar/common/protocol/PulsarHandler.java

@@ -37,7 +37,7 @@ public abstract class PulsarHandler extends PulsarDecoder {
     protected SocketAddress remoteAddress;
     private int remoteEndpointProtocolVersion = ProtocolVersion.v0.getValue();
     private final long keepAliveIntervalSeconds;
-    private volatile boolean waitingForPingResponse = false;
+    private boolean waitingForPingResponse = false;
     private ScheduledFuture<?> keepAliveTask;
 
     public int getRemoteEndpointProtocolVersion() {