macvim - remove xcode 4 check

macvim still won't comile under LLVM, but remove the explicit check
as we need a better solution for xcode 4 detection overall

Formula/macvim.rb

@@ -20,10 +20,6 @@ class Macvim <Formula
   depends_on 'cscope' if ARGV.include? '--with-cscope'
 
   def install
-    if "4.0" == xcode_version
-      opoo "MacVim may not compile under the Xcode 4 preview."
-    end
-
     # MacVim's Xcode project gets confused by $CC
     # Disable it until someone figures out why it fails.
     ENV['CC'] = nil