Remove oidc config from gradle plugin

[loosebazooka]

I think this config is unnecessary and is a bit complicated. In the future if a user truly needs this, we can add it back in. But weboidc should be configured from signing config. And ambient creds can be configured by modifying the env.

[loosebazooka]

@vlsi this mostly means getting rid of:

oidcClient {
        // oidcClient configuration should very rarely be configured, it should be
        // inferred from a sigstore deployment's config obtained from a TUF repository
        // with a default set of ambient credential providers
        gitHub {
            audience.set("sigstore")
        }
        web {
            clientId.set("sigstore")
            issuer.set("https://oauth2.sigstore.dev/auth")
        }
        // override the client config to a specific provider
        client.set(web)
        // or
        client(web)
    }