AK: Delete operator!() and operator bool() from the Nonnull pointers

Since NonnullRefPtr and NonnullOwnPtr cannot be null, it is pointless
to convert them to a bool, since it would always be true.

This patch makes it an error to null-check one of these pointers.

AK/NonnullOwnPtr.h

@@ -121,6 +121,9 @@ public:
     CALLABLE_WHEN(unconsumed)
     operator T*() { return m_ptr; }
 
+    operator bool() const = delete;
+    bool operator!() const = delete;
+
 private:
     void clear()
     {