Fix issue where GPU devices are denied access when MPS is enabled

The problem was that MPS and GPU plugins each have separate device
file records that point to the same file (duplicate device files are
normally rejected in gres.conf, so this is a special case). So when a
comprehensive GRES device list was assembled from each plugin in
gres_plugin_get_allocated_devices(), these files were being
double-counted, causing the issue.

The solution is to make this comprehensive GRES list unique by
omitting records with duplicate file paths.

Bug 7630