Remove all references to rpcuser, rpcpass, rpccert, and rpckey

[stasatdaglabs]

The above CLI flags are a relic from BTCD, and are no longer relevant.
Go over the entire codebase and remove all references to them.

[svarogg]

AFAICT rpcuser and rpcpass are already gone.

[cdnsteve]

rpccert and rpckey are still referenced but it looks like these might be useful in the auth story if we choose TLS as noted in #1893
Any context on the purpose of these, just old legacy? Better to remove and start over?

[someone235]

rpccert and rpckey are still referenced but it looks like these might be useful in the auth story if we choose TLS as noted in #1893 Any context on the purpose of these, just old legacy? Better to remove and start over?

These are legacy flags, but they can be useful for #1893. So it's the choice of who implements #1893 if to use them, or remove them and use something slightly different.