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`)

substrate/primitives/consensus/sassafras/Cargo.toml

@@ -3,7 +3,7 @@ name = "sp-consensus-sassafras"
 version = "0.3.4-dev"
 authors.workspace = true
 description = "Primitives for Sassafras consensus"
-edition = "2021"
+edition.workspace = true
 license = "Apache-2.0"
 homepage = "https://substrate.io"
 repository = "https://github.com/paritytech/polkadot-sdk/"

substrate/primitives/core/fuzz/Cargo.toml

@@ -2,6 +2,7 @@
 name = "sp-core-fuzz"
 version = "0.0.0"
 publish = false
+edition.workspace = true
 
 [lints]
 workspace = true

substrate/primitives/mixnet/Cargo.toml

@@ -4,7 +4,7 @@ name = "sp-mixnet"
 version = "0.4.0"
 license = "Apache-2.0"
 authors = ["Parity Technologies <admin@parity.io>"]
-edition = "2021"
+edition.workspace = true
 homepage = "https://substrate.io"
 repository = "https://github.com/paritytech/substrate/"
 readme = "README.md"

substrate/primitives/state-machine/src/basic.rs

@@ -33,7 +33,6 @@ use sp_trie::{empty_child_trie_root, LayoutV0, LayoutV1, TrieConfiguration};
 use std::{
 	any::{Any, TypeId},
 	collections::BTreeMap,
-	iter::FromIterator,
 };
 
 /// Simple Map-based Externalities impl.