Remove impls of ThirtyTwoByteHash for hashes types

These impls make using secp with hashes and bitcoin difficult during
upgrade, or if different versions of the libs get pulled in for any
reason. Since we do not use these impls in `rust-bitcoin` (we implement
the trait manually for sighash types) lets just remove them for now.

Note the implementations were a previous attempt at fixing this problem,
we refereed to it as the "dependency hole" because secp the repository
is outside of `rust-bitcoin` repo but `hashes` and `bitcoin` are in it.

Close: #673

src/lib.rs

@@ -198,21 +198,6 @@ pub trait ThirtyTwoByteHash {
     fn into_32(self) -> [u8; 32];
 }
 
-#[cfg(feature = "hashes")]
-impl ThirtyTwoByteHash for hashes::sha256::Hash {
-    fn into_32(self) -> [u8; 32] { self.to_byte_array() }
-}
-
-#[cfg(feature = "hashes")]
-impl ThirtyTwoByteHash for hashes::sha256d::Hash {
-    fn into_32(self) -> [u8; 32] { self.to_byte_array() }
-}
-
-#[cfg(feature = "hashes")]
-impl<T: hashes::sha256t::Tag> ThirtyTwoByteHash for hashes::sha256t::Hash<T> {
-    fn into_32(self) -> [u8; 32] { self.to_byte_array() }
-}
-
 /// A (hashed) message input to an ECDSA signature.
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct Message([u8; constants::MESSAGE_SIZE]);