Use = default for trivial special members

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

Change-Id: I254b0908a76d464131564b637cd0e42a6b03fb5a

internal/ceres/context_impl.h

@@ -47,11 +47,11 @@ namespace internal {
 
 class CERES_EXPORT_INTERNAL ContextImpl : public Context {
  public:
-  ContextImpl() {}
+  ContextImpl() = default;
   ContextImpl(const ContextImpl&) = delete;
   void operator=(const ContextImpl&) = delete;
 
-  ~ContextImpl() override {}
+  ~ContextImpl() override = default;
 
   // When compiled with C++ threading support, resize the thread pool to have
   // at min(num_thread, num_hardware_threads) where num_hardware_threads is