Removed is_file function call from NetteLoader

The purpose of this commit is **speed**. I think the is_file is not neccessary here. I've tried XHProf today and according to its results there was about 90 calls of the is_file function from NetteLoader and those calls took about 10ms in total (out of 200ms to load the entire page). That's a little too much in my opinion.

Can someone other confirm this?