test/hash.rb: remove Hash#freeze before Hash#rehash; ref mruby#6485

Author: matz

test/t/hash.rb

@@ -903,7 +903,6 @@ def h.default(k); self[k] = 1; end
     pairs1 = pairs.dup
     pairs1.delete([:_del, h.delete(:_del)])
     exp_pairs1 = pairs1.hash_for.to_a
-    h.freeze
     assert_same(h, h.rehash)
     assert_equal(exp_pairs1, h.to_a)
     assert_equal(exp_pairs1.size, h.size)