The T&P on the WH is required...it opens at MUCH higher temperatures and pressures to ensure the WH doesn't rupture. A steam explosion can literally flatten a house...you want it to work! A residential system, by code most places, says you need a pressure-reduction valve if your water exceeds 80psi. No idea what your pressure relief valve is set to, but it must be higher than your nominal max pressure, or you'll be wasting water all of the time to relieve it. Without an expansion tank, that relief valve is keeping your pressure down. A PRV may have an internal bypass, but it can only open when the house pressure exceeds the incoming water pressure - this means your pressure is not stable, and higher than code. Not all PRV's have bypass valves, either.
You do not need that relief valve. When the PRV starts to malfunction, it could help, but by then, it may no longer work. It could be left in, but they usually are more trouble than they are worth. The expansion tank just provides a place for that expanding water to go. Because it is compressing air, the amount of pressure rise is small for the volume of water needed, and your overall pressure will be pretty stable as long as the PRV is working and the ET is not waterlogged.