Possible to install unbranded android on HTC ONE?
It would be either very difficult or not possible at all.
Since Android is an Operating System, it is designed to fit varieties of devices, which differ in all kinds of modules that require specific drivers for themselves. Therefore if you want to create a clean Android build you must gather those drivers (and some additional configurations) to make it work properly on your device. Meanwhile, almost all 3rd-party ROMs are either manipulated from the official ones, or have extra configurations pre-applied, which both don't meet your requirement.