Remove Hash implementations for zkp types

An breaking change was inadvertently introduced with the release of
`rust-secp256k1-sys:0.4.2`. Anyone who had used `impl_array_newtype`
for type `T` and had also implemented `hash::Hash` on `T` before
version `0.4.2` would now come across a "conflicting implementations
error" on version `0.4.2`.

Author: luckysori

secp256k1-zkp-sys/src/zkp.rs

@@ -490,12 +490,6 @@ impl Default for Tag {
     }
 }
 
-impl hash::Hash for Tag {
-    fn hash<H: hash::Hasher>(&self, state: &mut H) {
-        self.0.hash(state)
-    }
-}
-
 impl From<[u8; 32]> for Tag {
     fn from(bytes: [u8; 32]) -> Self {
         Tag(bytes)
@@ -526,12 +520,6 @@ impl Default for PedersenCommitment {
     }
 }
 
-impl hash::Hash for PedersenCommitment {
-    fn hash<H: hash::Hasher>(&self, state: &mut H) {
-        state.write(&self.0)
-    }
-}
-
 /// A ring signature for the "whitelist" scheme.
 #[repr(C)]
 #[derive(Clone)]