Well, first, you can't do work in a rental property without 2 things.
- Landlord's permission to do the work AND
- Local government permission to do the work, which will only happen if the work is a) trivial or b) done by a licensed electrician.
That's the law.
"But this is such a minor thing. Why all the fuss?"
Oh, nothing minor about this! Dryer wiring, especially in older homes, often does not have a ground wire - it is wired hot-hot-neutral. It's not impossible to put 120V receptacles on that, but they need to be GFCI if they do. Sometimes it uses SE cable which has a bare neutral. Further, some older dryer wiring is actually wrong and can't be used for 240V, but can be used for 120V.
Dryer wiring in general is a hot mess, hard to keep Code legal, and really does not lend itself to tinkering by amateurs.
And we know the amateur brigade was here because of the mismatched breaker and receptacle. 20A breakers can feed 15A receptacles, but 30A breakers cannot.
...Maybe if the receptacle wiring is perfect.
But, if everything inside the receptacle was "tip top" -- 4-wire dryer feed (or metal conduit carrying the ground instead of a wire), one hot wire capped, neutral on neutral terminal and hot on hot terminal...
Then, I think you could limbo under the "trivial" category IF you do one super simple thing: Swap the 30A/240V breaker for a 20A/240V breaker. (15A sockets are allowed on 20A breakers if there is more than one socket on the circuit; almost any dual receptacle will do.)
That way you avoid complications with dangling wires in the panel, and getting the right blank cover plate (which is itself a real bug-hunt). The extra pole barely costs more than the blank cover plate, and is sturdier and easier to find.