Remove the file delete in favor of replacing the existed file.

This [comment](https://github.com/openscholar/openscholar/issues/4347#issuecomment-29632114) showed that there are problem with the file delete function and that there is a problem with the element validate function: the problem is that the element validate function has the same name as a hook and this caused for some errors when importing a file.

In this PR i replaced the name of the validate function and remove the file_delete function in favor of just replacing the file we uploaded and not creating a new one.