Fix import warnings

Recent rustc upgrade introduced some new warnings for incorrect imports,
fix them.

Author: tcharding

src/ecdsa/serialized_signature.rs

@@ -8,7 +8,6 @@
 //! serialized signatures and since it's a bit more complicated it has its own module.
 
 use core::borrow::Borrow;
-use core::convert::TryFrom;
 use core::{fmt, ops};
 
 pub use into_iter::IntoIter;

src/key.rs

@@ -3,7 +3,6 @@
 //! Public and secret keys.
 //!
 
-use core::convert::TryFrom;
 use core::ops::{self, BitXor};
 use core::{fmt, ptr, str};
 

src/schnorr.rs

@@ -12,9 +12,7 @@ use crate::ffi::{self, CPtr};
 use crate::key::{Keypair, XOnlyPublicKey};
 #[cfg(feature = "global-context")]
 use crate::SECP256K1;
-use crate::{
-    constants, from_hex, impl_array_newtype, Error, Message, Secp256k1, Signing, Verification,
-};
+use crate::{constants, from_hex, Error, Message, Secp256k1, Signing, Verification};
 
 /// Represents a schnorr signature.
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]