Revert "Refactor is_va_decl to use .map() and `.unwrap_or_default…

…()`."

Avoid code churn that doesn't have clear benefits.

This reverts commit 3866a89.

c2rust-transpile/src/translator/variadic.rs

@@ -25,12 +25,12 @@ macro_rules! match_or {
 impl<'c> Translation<'c> {
     /// Returns `true` iff `va_start`, `va_end`, or `va_copy` may be called on `decl_id`.
     pub fn is_va_decl(&self, decl_id: CDeclId) -> bool {
-        self.function_context
-            .borrow()
-            .va_list_decl_ids
-            .as_ref()
-            .map(|decls| decls.contains(&decl_id))
-            .unwrap_or(false)
+        let fn_ctx = self.function_context.borrow();
+        if let Some(ref decls) = fn_ctx.va_list_decl_ids {
+            decls.contains(&decl_id)
+        } else {
+            false
+        }
     }
 
     pub fn match_vastart(&self, expr: CExprId) -> Option<CDeclId> {