Remove unnecessary freeing functions (#40)

`rbs_node_destroy`, `rbs_hash_free`, `rbs_node_list_free` are only
calling each other recursively without any real freeing logic.

This is the result of previous efforts to allocate all nodes on the
arena. So we don't need these functions anymore.

Discovered while working on #41

Author: st0012

ext/rbs_extension/main.c

@@ -123,10 +123,7 @@ static VALUE parse_type_try(VALUE a) {
     arg->encoding
   );
 
-
-  VALUE ruby_ast = rbs_struct_to_ruby_value(ctx, type);
-  rbs_node_destroy((rbs_node_t *) type);
-  return ruby_ast;
+  return rbs_struct_to_ruby_value(ctx, type);
 }
 
 static rbs_lexer_t *alloc_lexer_from_buffer(rbs_allocator_t *allocator, VALUE string, rb_encoding *encoding, int start_pos, int end_pos) {
@@ -213,9 +210,7 @@ static VALUE parse_method_type_try(VALUE a) {
     arg->encoding
   );
 
-  VALUE ruby_ast = rbs_struct_to_ruby_value(ctx, (rbs_node_t *) method_type);
-  rbs_node_destroy((rbs_node_t *) method_type);
-  return ruby_ast;
+  return rbs_struct_to_ruby_value(ctx, (rbs_node_t *) method_type);
 }
 
 static VALUE rbsparser_parse_method_type(VALUE self, VALUE buffer, VALUE start_pos, VALUE end_pos, VALUE variables, VALUE require_eof) {
@@ -254,9 +249,7 @@ static VALUE parse_signature_try(VALUE a) {
     arg->encoding
   );
 
-  VALUE ruby_ast = rbs_struct_to_ruby_value(ctx, (rbs_node_t *) signature);
-  rbs_node_destroy((rbs_node_t *) signature);
-  return ruby_ast;
+  return rbs_struct_to_ruby_value(ctx, (rbs_node_t *) signature);
 }
 
 static VALUE rbsparser_parse_signature(VALUE self, VALUE buffer, VALUE start_pos, VALUE end_pos) {

include/rbs/ast.h

@@ -685,6 +685,4 @@ rbs_types_union_t *rbs_types_union_new(rbs_allocator_t *allocator, rbs_location_
 rbs_types_untyped_function_t *rbs_types_untyped_function_new(rbs_allocator_t *allocator, rbs_location_t *location, rbs_node_t *return_type);
 rbs_types_variable_t *rbs_types_variable_new(rbs_allocator_t *allocator, rbs_location_t *location, rbs_ast_symbol_t *name);
 
-void rbs_node_destroy(rbs_node_t *any_node);
-
 #endif