Remove Watcher only remove client-side watcher and remain server-side watcher leak

[tison]

See https://issues.apache.org/jira/browse/FLINK-33053.

While removeAllWatches removes all watches both client-side and
server-side, removeWatches only check the watcher existence and remove the
client-side one, remain the server-side watcher as is. Thus, it can leak if
no any further events.

Is it by design or we can make some better solution?

Best,
tison.