src: remove NonCopyableMaybe

I added this class in 823d86c in 2018
when we did not yet use `std::optional`. The last uses were removed in
5b9bf39, so remove it.

Refs: #24234
Refs: #55368
PR-URL: #58168
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Daeyeon Jeong <daeyeon.dev@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: tniessen

src/util.h

@@ -656,38 +656,6 @@ struct MallocedBuffer {
   MallocedBuffer& operator=(const MallocedBuffer&) = delete;
 };
 
-template <typename T>
-class NonCopyableMaybe {
- public:
-  NonCopyableMaybe() : empty_(true) {}
-  explicit NonCopyableMaybe(T&& value)
-      : empty_(false),
-        value_(std::move(value)) {}
-
-  bool IsEmpty() const {
-    return empty_;
-  }
-
-  const T* get() const {
-    return empty_ ? nullptr : &value_;
-  }
-
-  const T* operator->() const {
-    CHECK(!empty_);
-    return &value_;
-  }
-
-  T&& Release() {
-    CHECK_EQ(empty_, false);
-    empty_ = true;
-    return std::move(value_);
-  }
-
- private:
-  bool empty_;
-  T value_;
-};
-
 // Test whether some value can be called with ().
 template <typename T, typename = void>
 struct is_callable : std::is_function<T> { };