Replacing 240V outlet with 120V
It can be done, but you'll have to swap out the breaker.
A &quot;standard&quot; US receptacle is rated for 125 volts, and 15 or 20 amperes. So you'll be going from a 30 ampere double pole breaker, to a 15 or 20 ampere single pole breaker.  You could install a 15 or 20 ampere double pole breaker, but since you're not using the second line there's no point.
In the panel
Install the single pole breaker, and connect one of the ungrounded (hot) conductors to it. Cap off the other ungrounded (hot) conductor, using a twist-on wire connector or other approved means. The grounded (neutral) and grounding conductors should already be terminated properly, so you shouldn't have to do anything with them.
Since you've gone from a double pole breaker to a single pole, there's going to be a gaping hole in the panel cover. You'll have to cover it using a listed and labeled breaker filler plate.
In the bathroom
If You're installing a 15 ampere circuit, you'll have to use a short bit of 14 or 12 AWG wire to connect the branch circuit conductor to the receptacle. Cut a piece of 14 or 12 AWG wire about 6&quot; long, and strip both ends. Connect one end to the 10 AWG conductor coming into the box, using a twist-on wire connector of appropriate size (or other approved means). Connect the other end to the screw terminal of the receptacle.  Repeat the same process for the grounded (neutral) conductor, and grounding conductor.
If you're installing a 20 ampere circuit, a 20 ampere receptacle may accept 10 AWG wire. Check the manufacturer's documentation to determine what size conductors can be connected to the device. If it does accept 10 AWG, simply connect the conductors to the screw terminals on the receptacle. If not, you'll have to follow the procedure from above, using only 12 AWG conductors.
Once the receptacle is wired up, you can cap off the unused wire using a twist-on wire connector (or other approved means).
GFCI Protection
Since receptacle is going to be in a bathroom, you'll have to provide GFCI protection. This can be done via a GFCI breaker, or a GFCI receptacle.
Notes:
If you don't feel comfortable working in the panel, don't hesitate to contact a local licensed Electrician.
Before beginning this project, make sure you know which towel bar you'll be installing. Read and understand all the documentation that comes with it, and determine what size circuit you'll need to supply it.