You only need RX, TX and gnd for basic communication between 2 devices. If you want to do all kinds of fancy handshaking in your software, then you need CTS and RTS as well.
Be VERY careful when drilling out pads. If your drill is too large or not sharp enough, you may rip the whole trace that is attached to that pad, off the board. I doubt the holes are filled with solder, but if they are, I'd just use some desoldering braid. If you have access to a vacuum solder sucker, definitely use that.
"Anything that can possibly go wrong, does" -M