Hi,
First of all, you need to know if this happened outside the 5 year period from the date you have your interview. It is extremely important in this case.
2- How did you register? was there a box for you to check YES or No for citizenship? Very very very important. If you claimed to be a citizen on purpose to obtain some sort of benefit, then you are in big trouble, but if this was intentionally, which the Immigration Officer has to determine, then you will be fine.
3- Get all your voting records that shows when and how many times you voted
4- remove your name from voting registry (which you did already)
5- write a detailed letter that explains the circumstances and convince the IO that you did not do this on purpose. Show your good moral, your clean records, your job title, your family, church involvement and how you are and have always been a good person who contribute to the society.
Do not believe lawyers who want to charge you $1000 for a letter you can write yourself.
You will be fine man don't worry too much if you did not do this on purpose. This is a great country and they know people make mistakes
I forgot to mention that you must check YES in the bos where they ask you if you have ever registered or voted? Vey very very very very important. If you don't, you will be taking a 50/50 chance, they may never find out and they may find out and have you deported for two things, lying and claiming to be a US citizen by voting.