brew unlink: don't unlink an unlinked formula

This caused weird issues in the case that multiple formulae provide
the same file.

Library/Homebrew/cmd/unlink.rb

@@ -3,6 +3,7 @@ module Homebrew extend self
     raise KegUnspecifiedError if ARGV.named.empty?
 
     ARGV.kegs.each do |keg|
+      return if !keg.linked?
       print "Unlinking #{keg}... "
       puts "#{keg.unlink} links removed"
     end