simplify some trivial rb_data_type_t callbacks

* process.c (free_exec_arg): remove
  (memsize_exec_arg): ptr is never NULL
  (exec_arg_data_type): use RUBY_TYPED_DEFAULT_FREE

* variable.c (autoload_i_free): remove
  (autoload_data_i_type): use RUBY_TYPED_DEFAULT_FREE
  (autoload_memsize): ptr is never NULL

* vm_backtrace.c (location_free): remove
  (location_mark): ptr is never NULL
  (location_data_type): use RUBY_TYPED_DEFAULT_FREE
  (backtrace_mark): ditto
  (backtrace_free): ditto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47572 b2dd03c8-39d4-4d8f-98ff-823fe69b080e