chore: remove original return from aztec fns (#4804)

The following function:

```rust
#[aztec(private)]
fn do_work_conditionally() -> u64 {
  let condition = storage.flag;
  if condition {
     2
  } else {
     42
  }
}
```

Will be transformed by the macro, adding a push with the whole if/else
expression to `context.return_values`...but without removing the
statement in the first place, leading to duplicate code execution.

This PR forces the removal of the last function statement in case it is
replaced by a push to the context.

noir/aztec_macros/src/lib.rs

@@ -648,6 +648,10 @@ fn transform_function(
 
     // Abstract return types such that they get added to the kernel's return_values
     if let Some(return_values) = abstract_return_values(func) {
+        // In case we are pushing return values to the context, we remove the statement that originated it
+        // This avoids running duplicate code, since blocks like if/else can be value returning statements
+        func.def.body.0.pop();
+        // Add the new return statement
         func.def.body.0.push(return_values);
     }
 
@@ -1255,15 +1259,13 @@ fn create_avm_context() -> Result<Statement, AztecMacroError> {
 /// Any primitive type that can be cast will be casted to a field and pushed to the context.
 fn abstract_return_values(func: &NoirFunction) -> Option<Statement> {
     let current_return_type = func.return_type().typ;
-    let len = func.def.body.len();
-    let last_statement = &func.def.body.0[len - 1];
+    let last_statement = func.def.body.0.last();
 
     // TODO: (length, type) => We can limit the size of the array returned to be limited by kernel size
     // Doesn't need done until we have settled on a kernel size
     // TODO: support tuples here and in inputs -> convert into an issue
-
     // Check if the return type is an expression, if it is, we can handle it
-    match last_statement {
+    match last_statement? {
         Statement { kind: StatementKind::Expression(expression), .. } => {
             match current_return_type {
                 // Call serialize on structs, push the whole array, calling push_array