* gc.c (unlink_heap_slot): fix memory leak.
  free slot itself at free_heap_slot().
  Reproduce-able code is here:
  N1 = 100_000; N2 = 1_000_000
  N1.times{ary = []; N2.times{ary << ''}}
  Maybe this problem is remaining in Ruby 2.0.0.
* gc.c (unlink_heap_slot): remove not working code.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42022 b2dd03c8-39d4-4d8f-98ff-823fe69b080e