Fix rm -r race condition

When starting rich_cms removes the target dir before creating it again
and copying over assets. This can lead to race conditions if you have 
multiple processes starting up, with concurrent corncobs for example.

lib/rich/cms/engine.rb

@@ -58,7 +58,6 @@ def copy_images
           source_dir = File.join File.dirname(__FILE__), "..", "..", "..", "assets", "images", "."
           target_dir = File.join Rails.root, "public", "images", "rich", "cms"
 
-          FileUtils.rm_r    target_dir if File.exists? target_dir
           FileUtils.mkdir_p target_dir
           FileUtils.cp_r    source_dir, target_dir
         end