WOW as always, ask an electrical question and there will be several answers, many accurate some maybe confusing. Here's the deal in my professional opinion albeit rusty as I'm long retired:
QUESTION: "I have 220V going to plug for AC. The plug isn't in use any longer. Can the wires to that plug be used for 110V?"
1) Not being there I would say there's a good chance the 240 receptacle has two hots, L1 & L2 plus a typical bare/green EQUIPMENT GROUNDING CONDUCTOR (That's NOT the same as a Neutral). The two hots L1 & L2 would be 240 Volts line to line or 120 either Line to a Neutral.
2) Ifffffffffff ?? that's indeed the case, you should NOT (if safety and the NEC matters) use one hot 120 volt leg and substitute an actual EQUIPMENT GROUNDING CONDUCTOR for the Neutral (NEUTRAL IS NOTTTTTT THE SAME AS THE EQUIPMENT GROUNDING CONDUCTOR) that's a safety and National Electrical Code (NEC) violation. YES Billy Bob and Bubba it will "work" using one hot leg and the equipment grounding conductor substituted for the Neutral. If there are three wires from panel to the receptacle they can, however, be re wired and re configured as described below correctly using a Hot, a Neutral, and a third safety wire equipment grounding conductor.
3) If its a 240 circuit it will be connected to a 240 volt two pole circuit breaker, but for 120 a single pole 120 volt breaker should be used.
4) In the event you want to install a 120 volt 15 amp NEMA 5-15R 2 pole 3 wire grounding receptacle there and want to re wire and reconfigure it can indeed be done as follows using three wires: A 120 Volt two pole two wire receptacle that has no equipment ground requires only two wires.
Use ONE of the two Hots, L1 OR L2, it will be wired to a 120 volt 15 amp single pole circuit breaker in the panel NOT one leg of the 240 volt two pole breaker is my suggestion even if that still works,,,,,,,,,,,,,,Use/run a wire for the Neutral which connects to the Neutral Buss in your panel,,,,,,,,,,,,,Use/run a wire (often insulated sometimes bare, typically Green if insulated) from the grounding terminal on the receptacle to the GROUND BUSS in the panel.
NOTE on some 120/240 Volt main distribution (NOT a sub panel) panels there are separate Neutral and Ground busses bonded together while on some panels there is only one common Neutral and Ground buss where all the white Neutrals and bare/green grounding conductors attach.
NOTE if its really old two wire system there are Hots and Neutrals ran to 120 volt 2 pole receptacles that don't have any third safety wire grounding terminal.
NOTE if its a more modern 120/240 Volt Single Phase Three Wire distribution panel there are three wires ran to NEMA 5 -15R 120 Volt 2 Pole 3 wire grounding receptacles, being a Hot, a Neutral and an equipment grounding conductor.
NOTE if its a metal junction/receptacle box, it also needs bonded to the equipment grounding conductor.
SOOOOOOOOOOOOO yes it can be done and it requires three proper wires (Hot, Neutral, Ground) for a 120 Volt 15 Amp 2 pole 3 wire NEMA 5-15R grounding receptacle
If in doubt where life and fire safety is concerned consult trained professional electricians and engineers, the NEC and local authority NOT an internet forum and not what I say, I'm long retired n rusty as a power distribution design engineer.