refactor: remove default contraint in merkle tree trait (#1004)

Author: MarcosNicolau

crates/crypto/src/merkle_tree/traits.rs

@@ -5,7 +5,7 @@ use rayon::prelude::{IntoParallelRefIterator, ParallelIterator};
 /// A backend for Merkle trees. This defines raw `Data` from which the Merkle
 /// tree is built from. It also defines the `Node` type and the hash function
 /// used to build parent nodes from children nodes.
-pub trait IsMerkleTreeBackend: Default {
+pub trait IsMerkleTreeBackend {
     type Node: PartialEq + Eq + Clone + Sync + Send;
     type Data: Sync + Send;