Remove comment for fixed bug at r2579

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2585 f4f36d40-9719-0410-9868-dde021a23eda
remove import obsoleted header file "ruby/backward/rubysig.h"

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2586 f4f36d40-9719-0410-9868-dde021a23eda