Fix accidentally introduced ambiguity

Author: westonpace

cpp/src/arrow/compute/exec/exec_plan.h

@@ -63,12 +63,12 @@ class ARROW_EXPORT ExecPlan : public std::enable_shared_from_this<ExecPlan> {
 
   ARROW_DEPRECATED("Deprecated in 11.0.0. Use version that takes ExecContext by value.")
   static Result<std::shared_ptr<ExecPlan>> Make(
-      QueryOptions options, ExecContext* exec_context = default_exec_context(),
+      QueryOptions options, ExecContext* exec_context,
       std::shared_ptr<const KeyValueMetadata> metadata = NULLPTR);
 
   ARROW_DEPRECATED("Deprecated in 11.0.0. Use version that takes ExecContext by value.")
   static Result<std::shared_ptr<ExecPlan>> Make(
-      ExecContext* exec_context = default_exec_context(),
+      ExecContext* exec_context,
       std::shared_ptr<const KeyValueMetadata> metadata = NULLPTR);
 
   ExecNode* AddNode(std::unique_ptr<ExecNode> node);