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`)

cumulus/client/consensus/aura/src/collator.rs

@@ -55,7 +55,7 @@ use sp_runtime::{
 };
 use sp_state_machine::StorageChanges;
 use sp_timestamp::Timestamp;
-use std::{convert::TryFrom, error::Error, time::Duration};
+use std::{error::Error, time::Duration};
 
 /// Parameters for instantiating a [`Collator`].
 pub struct Params<BI, CIDP, RClient, Proposer, CS> {

cumulus/client/consensus/aura/src/collators/basic.rs

@@ -48,7 +48,7 @@ use sp_inherents::CreateInherentDataProviders;
 use sp_keystore::KeystorePtr;
 use sp_runtime::traits::{Block as BlockT, Header as HeaderT, Member};
 use sp_state_machine::Backend as _;
-use std::{convert::TryFrom, sync::Arc, time::Duration};
+use std::{sync::Arc, time::Duration};
 
 use crate::collator as collator_util;
 

cumulus/client/consensus/aura/src/collators/lookahead.rs

@@ -67,7 +67,7 @@ use sp_inherents::CreateInherentDataProviders;
 use sp_keystore::KeystorePtr;
 use sp_runtime::traits::{Block as BlockT, Header as HeaderT, Member};
 use sp_timestamp::Timestamp;
-use std::{convert::TryFrom, sync::Arc, time::Duration};
+use std::{sync::Arc, time::Duration};
 
 use crate::collator::{self as collator_util, SlotClaim};