Whether you complete the master's program or not is up to you. One D-school told me I didn't have to complete my MS program, as it was not a requirement for graduation. I'm going to complete it, as I only have a semester left. I am sure you can withdraw without consequences. However, make sure all parties involved are aware of your plans. If you are getting through grad school on a teaching/research assistant stipend, you should most definitely inform your department chair/advisor about your recent acceptance so they know some funds are now available.
Also, ask yourself some questions first. Why did you pursue the master's program in the first place? What is type of program is it (MA, MS, thesis/non-thesis, the type of major/concentration)? Can I save a lot of money be not going back this Spring (uh, yeah). Was it just to beef up the resume or were you looking for something more after undergrad? What is your intention following dental school (hard to say, I know, but level with me here)? If GP is your goal why waste time with something your heart isn't into (just my two cents)? On the other hand, maybe it might come in handy later in life--depending on what it is of course (take an MA in underwater basketweaving for example-probs not going to help ya out later, aside form impressing the ladies of course). Do graduate degrees look good to a residency program? Not sure. Maybe? Depends on your performance in D-school (right?). Also, would/could you even defer? I don't know. Would you want to?
All in all, I'm sure you can forget the master's program if dental school was your primary intention from the start. Just don't leave the faculty who considered your application for this program out high and dry. Something like, "it's not you, it's me. I was just using you from the start" sort of thing. Hope that helps and some more people weigh in as well.