I've yet to see anyone ask this question. My bet is there is something you are trying to do but didn't reveal.
I installed Phoenix O/S as a test for something [didn't work, no matter]. Uninstalling its program didn't stop it from trying to dual-boot on switching on. I found its .efi in the registry, used Admin., etc. to delete it, but it came back.
I've tried everything from PF10 on Boot to examining the Disk partitions. It is invisible!