[cleanup][broker] Remove unused cache executor in PulsarService

[lifepuzzlefun]

Motivation

Remove unused cache executor in PulsarService

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[AnonHxy]

LGTM

[michaeljmarshall]

This is technically a breaking change for pulsar extensions. I don't know of any extensions using it, but it seems risky to just remove the method. @BewareMyPower - what is your opinion on the right way to handle these kinds of methods that could be used by extensions?

[BewareMyPower]

A better way might be adding the @Deprecated annotation to all these public methods first, then the extension developer could be aware that these methods could be removed in future.

But I think it's reasonable to remove unused methods in Pulsar, the extension should not depend on public methods from a class (rather than an interface) too much. Public methods are added too casually. Just like #20537, authors might just add a getter (e.g. NamespaceBundles#getNsname) for a very limited use case and they might not realize they have added interfaces that could bring breaking changes if they are removed.

If we want to guarantee all public methods in classes that should not be removed directly, we should guarantee any public method should not be added without any care. Otherwise, there could be too many public methods that require PIPs to remove them.

However, if so, it could be a burden to Pulsar developers that each new public method requires a PIP.