Officials in Somerville are moving to get rid of an antiquated municipal ordinance that bans people from singing or playing an instrument in a public place without a police detail. The city’s ordinance for street performers has been on the books in 1963, but Alderwoman Rebekah Gewirtz has sought its removal since last September, after receiving a complaint from street performers who were told by police that they needed a permit. Gewirtz has argued that the ordinance may violate the First Amendment. The aldermen’s Legislative Matters Committee approved an amendment to the city ordinance in a meeting last week. The proposal is expected to be reviewed by the Board of Aldermen as a whole on Thursday.