configure.in: remove no longer used variable

* configure.in (unexpand_shvar): remove variable which is no longer
  used since r39506.  [Bug #7959]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39514 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-02-27

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39515 b2dd03c8-39d4-4d8f-98ff-823fe69b080e