Remove file checking from ResourceCaches.java

Hi,

I have found that a ContentLoader is responsible for checking the existence of the File, so this check in ResourceCaches is somehow redundant. Also, removing this line can help the ContentLoader to load file from the different location.
(It should not be a bug, actually.)

Here's the pull request, if you can merge the patch from Github:

https://github.com/zkoss/zk/pull/1