I have a shimano acera crankset (3x) that is 28-38-48 with a 8 speed rear derail. Now I was going to convert this bike into a SS but decided not to mess with it. Can I take off the middle chainring (38) and replace it with a 44 teeth chainring instead without any messing/adjusting with the front derail.? (I'll just be using the middle chainring)
You'll have to 'mess' minimally with the FD in a way that you'll have to adjust the min/max screws to lock the FD in position and (better) remove the cable. Leave the derailleur in place as it will act as a chain-guide.
Yes, that is possible. Apart from adjustments to the front derailleur in @Carel's answer, you will have to purchase some shorter single chainring bolts as your double chainring bolts will be too long.