remove unnessisary use statements due to 2021 core prelude (#4183)

Some traits are already included in the 2021 prelude and so shouldn't be
needed to use explicitly:

use `convert::TryFrom`, `convert::TryInto`, and `iter::FromIterator` are
removed.

( https://doc.rust-lang.org/core/prelude/rust_2021/ )

No breaking changes or change of functionality, so I think no PR doc is
needed in this case.

(Motivation: Removes some references to `sp-std`)

polkadot/node/core/bitfield-signing/src/lib.rs

@@ -38,7 +38,7 @@ use polkadot_node_subsystem::{
 use polkadot_node_subsystem_util::{self as util, Validator};
 use polkadot_primitives::{AvailabilityBitfield, CoreState, Hash, ValidatorIndex};
 use sp_keystore::{Error as KeystoreError, KeystorePtr};
-use std::{collections::HashMap, iter::FromIterator, time::Duration};
+use std::{collections::HashMap, time::Duration};
 use wasm_timer::{Delay, Instant};
 
 mod metrics;

polkadot/node/network/bitfield-distribution/src/tests.rs

@@ -40,7 +40,7 @@ use sp_core::Pair as PairT;
 use sp_keyring::Sr25519Keyring;
 use sp_keystore::{testing::MemoryKeystore, Keystore, KeystorePtr};
 
-use std::{iter::FromIterator as _, sync::Arc, time::Duration};
+use std::{sync::Arc, time::Duration};
 
 const TIMEOUT: Duration = Duration::from_millis(50);
 macro_rules! launch {

polkadot/node/network/collator-protocol/src/collator_side/mod.rs

@@ -16,7 +16,6 @@
 
 use std::{
 	collections::{HashMap, HashSet},
-	convert::TryInto,
 	time::Duration,
 };
 

polkadot/node/network/collator-protocol/src/validator_side/mod.rs

@@ -20,9 +20,7 @@ use futures::{
 use futures_timer::Delay;
 use std::{
 	collections::{hash_map::Entry, HashMap, HashSet},
-	convert::TryInto,
 	future::Future,
-	iter::FromIterator,
 	time::{Duration, Instant},
 };
 use tokio_util::sync::CancellationToken;

polkadot/node/network/statement-distribution/src/legacy_v1/tests.rs

@@ -55,7 +55,7 @@ use sp_application_crypto::{sr25519::Pair, AppCrypto, Pair as TraitPair};
 use sp_authority_discovery::AuthorityPair;
 use sp_keyring::Sr25519Keyring;
 use sp_keystore::{Keystore, KeystorePtr};
-use std::{iter::FromIterator as _, sync::Arc, time::Duration};
+use std::{sync::Arc, time::Duration};
 use util::reputation::add_reputation;
 
 // Some deterministic genesis hash for protocol names

polkadot/node/subsystem-types/src/messages/network_bridge_event.rs

@@ -14,7 +14,7 @@
 // You should have received a copy of the GNU General Public License
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
 
-use std::{collections::HashSet, convert::TryFrom};
+use std::collections::HashSet;
 
 pub use sc_network::ReputationChange;
 pub use sc_network_types::PeerId;