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/problem.cc

@@ -46,7 +46,7 @@ Problem::Problem(const Problem::Options& options)
 // Not inline defaulted in declaration due to use of std::unique_ptr.
 Problem::Problem(Problem&&) = default;
 Problem& Problem::operator=(Problem&&) = default;
-Problem::~Problem() {}
+Problem::~Problem() = default;
 
 ResidualBlockId Problem::AddResidualBlock(
     CostFunction* cost_function,