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

signer/src/crypto/derive_junction.rs

@@ -3,7 +3,6 @@
 // see LICENSE for license details.
 
 use codec::{Decode, Encode};
-use polkadot_sdk::sp_crypto_hashing;
 
 // This code is taken from sp_core::crypto::DeriveJunction. The logic should be identical,
 // though the API is tweaked a touch.