feat(mempool): implement remove method for transaction queue (#412)

crates/mempool/src/transaction_queue.rs

@@ -55,6 +55,15 @@ impl TransactionQueue {
     pub fn _get_nonce(&self, address: ContractAddress) -> Option<Nonce> {
         self.address_to_tx.get(&address).map(|tx| tx.nonce)
     }
+
+    /// Removes the transaction of the given account address from the queue.
+    /// This is well-defined, since there is at most one transaction per address in the queue.
+    pub fn _remove(&mut self, address: ContractAddress) -> bool {
+        if let Some(tx) = self.address_to_tx.remove(&address) {
+            return self.queue.remove(&tx.into());
+        }
+        false
+    }
 }
 
 /// Encapsulates a transaction reference to assess its order (i.e., priority).