That conical part ( the middle of the stem) is your temperature control. It turns independently within the stem itself. You don't remove it. Leave it as it is.
Is that screw on side of that conical part or in the middle / inside of it ?
Can you snap clearer and closer photo ? Also from 45 degree view ? Put your camera on Macro
Ok, there may be different solution to your problem. These valves had notorious problem with the handle not holding tight on that conical part. As you keep adjusting hot/cold water, it slowly slipped towards cold water side cutting your hot water in the process. After some time, you will end up with no hot water. You will have to remove the handle, readjust the conical stem - and you have hot water again. Please, do this test for me:
1. Turn water on with the lever handle. Do not install the Temperature Control Knob/handle. Let water flow through shower head
2. Take small channel locks and turn that conical part all the towards hot water, as far as it goes. Let it run for few minutes. See if you get hot water.
Let me know if it worked...