Yes, you are safe to delete the old catalog. I would suggest that you create a backup of the new one before you delete the old, unless you have other backups.
I suggest you also keep an external backup copy of the original LR5 catalog for at least a few months until you are absolutely certain there are no issues with your LR5 edited images. There was an issue with LR4 to LR5 and then LR5 to LR5.2 catalog conversions that caused feathering to be lost on all Spot Removal Tool applications and 'Constrain to Crop' to be lost on all images with Transforms. So far for me LR 6/CC looks good, but it will take reviewing of your previously edited LR5 images to be sure. Better safe than sorry! I had to use AnyFilter plugin to locate the affected LR4 images after upgrading to LR5 even though I had a backup copy of my LR catalog. Why? By the time I discovered the conversion issue I had already imported and edited thousands of new image files.
Moral: Anytime you update or upgrade an application review your previous work files with the new version before making extensive edits to existing and new image files.
Thanks very much to all who replied - I do have the old catalog backed up on external drive so will go ahead and delete from working drive.