Use = default for trivial special members

Applied changes correspond to clang-tidy fixes
stemming from the modernize-use-equals-default check.

Change-Id: I254b0908a76d464131564b637cd0e42a6b03fb5a

include/ceres/sized_cost_function.h

@@ -61,7 +61,7 @@ class SizedCostFunction : public CostFunction {
     *mutable_parameter_block_sizes() = std::vector<int32_t>{Ns...};
   }
 
-  ~SizedCostFunction() override {}
+  ~SizedCostFunction() override = default;
 
   // Subclasses must implement Evaluate().
 };