Fix realpath check in RdfImporter

[zerocrates]

It's currently causing issues on Windows because we use mixed slashes (PHP doesn't care, but realpath corrects them), see https://forum.omeka.org/t/errors-after-installing-omeka-s/3088/6

It's also possible it would cause issues with legitimate symlinks. The idea here with these "realpath" type checks usually is to ensure that there's no attempt to give, for example, a filename within a directory that attempts to traverse down the tree and reference something elsewhere on the filesystem. Here we're not attempting to be restrictive in that way, and the Importer's facility for reading a file at a given path isn't directly exposed anywhere, so I don't think the check here really is providing any value.