Use iter().flatten() instead of if let Some

Clippy emits:

  warning: unnecessary `if let` since only the `Some` variant of the
  iterator element is used

As suggested use `iter().flatten()` to get only the non-none nodes.

src/util/taproot.rs

@@ -442,11 +442,9 @@ impl TaprootBuilder {
 
     /// Checks if the builder has hidden nodes.
     pub fn has_hidden_nodes(&self) -> bool {
-        for node in &self.branch {
-            if let Some(node) = node {
-                if node.has_hidden_nodes {
-                    return true
-                }
+        for node in self.branch.iter().flatten() {
+            if node.has_hidden_nodes {
+                return true
             }
         }
         false