Remove unused private from folly/test/SynchronizedTest.cpp

Summary:
`-Wunused-private-field` has identified an unused private field. This diff removes it.

If the code compiles, this is safe to land.

Reviewed By: dtolnay

Differential Revision: D70933435

fbshipit-source-id: b0c776f4c0f623371de98a89667fc4168d37d48e

Author: r-barnes

folly/test/SynchronizedTest.cpp

@@ -1015,8 +1015,8 @@ class TestStruct {
   TestStruct(int a, int b) : a_{a}, b_{b} {}
 
  private:
-  int a_{0};
-  int b_{0};
+  [[maybe_unused]] int a_{0};
+  [[maybe_unused]] int b_{0};
 };
 } // namespace