Move Tree destructor to header file (#3417)

~ Added 'noexcept' specifier and defaulted desctructor.
Co-authored-by: Lucas DAVID <lucas@isdom.isoft.fr>

include/LightGBM/tree.h

@@ -38,7 +38,7 @@ class Tree {
   */
   Tree(const char* str, size_t* used_len);
 
-  ~Tree();
+  ~Tree() noexcept = default;
 
   /*!
   * \brief Performing a split on tree leaves.

src/io/tree.cpp

@@ -48,9 +48,6 @@ Tree::Tree(int max_leaves, bool track_branch_features)
   max_depth_ = -1;
 }
 
-Tree::~Tree() {
-}
-
 int Tree::Split(int leaf, int feature, int real_feature, uint32_t threshold_bin,
                 double threshold_double, double left_value, double right_value,
                 int left_cnt, int right_cnt, double left_weight, double right_weight, float gain,