keg: delete bad tap opt non-symlink directories.

But don't delete them if they are a symlink to a directory i.e. a normal
opt link.

Handles the issue mentioned in
1651647a3d (r28360427)

Library/Homebrew/keg.rb

@@ -238,7 +238,9 @@ class Keg
 
     if tap
       bad_tap_opt = opt/tap.user
-      FileUtils.rm_rf bad_tap_opt if bad_tap_opt.directory?
+      if !bad_tap_opt.symlink? && bad_tap_opt.directory?
+        FileUtils.rm_rf bad_tap_opt
+      end
     end
 
     aliases.each do |a|