rpc: Remove deprecated helper function

[steviez]

Summary of Changes

getConfirmedSignaturesForAddress was removed in v2.0. This changes removes get_confirmed_signatures_for_address(), which was a helper function for that RPC method. This was seemingly just missed in the commit that removed the deprecated functions

In #1809, we can see that the only caller of this function was removed. clippy probably just missed this function as unused due to pub:
https://github.com/anza-xyz/agave/pull/1809/files#diff-b55d9bcb5d2e7eae09fd352edcc765f9c2a4c05b97e9847582071d35850e8671L4735-L4762

[mergify]

If this PR represents a change to the public RPC API:

1. Make sure it includes a complementary update to rpc-client/ (example)
2. Open a follow-up PR to update the JavaScript client @solana/web3.js (example)

Thank you for keeping the RPC clients in sync with the server API @steviez.

[steveluscher]

This is still a method in @solana/web3.js@1 but it no longer relies on calling this puppy. It emulates it.

https://github.com/solana-labs/solana-web3.js/blob/4e9988cfc561f3ed11f4c5016a29090a61d129a8/src/connection.ts#L5446-L5505