Refactor mrb_define_class() method in class.c.c

A temporary c variable is redundancy.
<pre>
c = mrb_define_class_id(mrb, mrb_intern(mrb, name), super);
return c;
</pre>

Set a value in return statement.
<pre>
return mrb_define_class_id(mrb, mrb_intern(mrb, name), super);
</pre>