deps: remove polkadot-sdk umbrella crate (#1926)

* deps: get rid of polkadot-sdk umbrella crate

* fix nits

* Update subxt/src/backend/mod.rs

* Update subxt/src/events/events_client.rs

* Update metadata/src/utils/validation.rs

* cargo clippy fix

* fix ui tests

subxt/src/backend/mod.rs

@@ -382,7 +382,6 @@ mod test {
     use crate::backend::StorageResponse;
     use core::convert::Infallible;
     use futures::StreamExt;
-    use polkadot_sdk::sp_core;
     use primitive_types::H256;
     use rpc::RpcClientT;
     use std::collections::{HashMap, VecDeque};