Yes - you can leave it behind but at some point you will need to reformat and break the fusion into the 2 separate drives that actually there. Please backup first as this is a data destructive operation.

I just upgraded my late 2015 iMac with a 1TB Samsung 860 EVO and did not use a sensor. No fan or heat issues over the last week of use. Please note this Samsung drive is not the primary workhorse on the machine as I also upgraded my PCI ssd and use that as the primary drive. I have read on other sites that this model iMac does not need the sensor but I am not 100% on that.

First: During reassembly, I highly recommend you connect these cables and do a test boot BEFORE you remove the protective tape on the adhesive strips. That way you can fix any issues(like a damaged cable) if something is not right and you have not re-glued the screen back down which will cause a problem. If you have a second person and everything is working after test boot, you can just leave the cables connected, lift the bottom of the screen and have 2nd person pull the protective tape on the adhesive strips and you can set the screen in place. Second I damaged the video data cable during reassembly on my late 2015 iMac- I did not get the connector straight and I damaged the wires on the cable. I recommend you have someone else hold the screen while you attach the cables - they are fragile. Luckily, the data cable was easily replaced for only about $20. If you must do this by yourself, just be careful I highly recommend have 2 people at this step during reassembly.