PHPCS: restore Generic.CodeAnalysis.UnusedFunctionParameter smell

[justlevine]

Per #2924 (comment)_ we should remove Generic.CodeAnalysis.UnusedFunctionParameter from our list of excluded PHPCS rules and any unused parameters from our method signatures.

Pros

• improves readability (it's clear what parameters are actually necessary)
• reduces code debt and future-compat ( lot easier to add a parameter than remove one that's not even in use).
• (premature) performance optimization (since params don't always get garbage collected well).
• brings us in line with the ecosystem coding standards.
• (...suggest more in the comments)

Cons

• Reduces self-documentation (since functions now show what is being passed to the method and no longer what can be passed, live code is no longer a "complete" example.
• (...suggest more in the comments)

[justlevine]

As of #2933 unused parameters inside a 'resolve' callback function have been removed.

Remaining smells in this sniff are for the most part limited to actual methods (either public or a protected method on an abstract class). These would be technically breaking to remove (even if IRL most of them are only intended to be used internally, e.g. BuiltInType::register_type( TypeRegistry $registry ) ).

A path forward would be to deprecate the unused parameters and remove them entirely in a future (breaking) release. This is the "proper" thing to do, but is a bit of an undertaking comes with significant code debt.

Alternatively, we could skip the deprecation step, and leave the smell silenced until it can be remediated in v2.0.

@jasonbahl thoughts?

[jasonbahl]

@justlevine I think silencing for now works.

I'm not opposed to the intermediate deprecation path and full removal in a breaking release later.