tempfile.rb: undefine finalizer on unlink

* lib/tempfile.rb (Tempfile#unlink): finalizer is no longer needed
  after unlinking.  patched by by normalperson (Eric Wong) at
  [ruby-core:56521] [Bug #8768]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43110 b2dd03c8-39d4-4d8f-98ff-823fe69b080e