Glenn Fleishman, writing for Macworld:
Deleting a hard link in one place leaves all the other references intact. When the number of hard links drops to just one, you’ve just got a file! No hard links at all. And deleting that one reference, the file itself, truly does throw the file in the trash. Thus, delete your iPhoto Library, and—ostensibly—you won’t delete any files shared by Photos through hard links.
Having said all that, please make a complete backup of both your iPhoto and Photos libraries before deleting the iPhoto Library. You should be able to toss it and lose nothing, but I’m not so blithe as to suggest you whistle while you’re emptying the trash and assume all is well.
A great tip for anyone who has fully transitioned from iPhoto to Photos on the Mac.