The box office boon that Usher brought to “Chicago” has been cut a little short.The pop star-thesp, who goosed sales at the 10-year-old revival when he stepped into the show in August, is unable to finish his engagement in the show due to strep throat. The illness has sidelined him since Oct. 6. He was skedded to appear through Saturday. Because the show is offering refunds to disappointed Usher fans, a drop at the B.O. looks likely. Grosses last week dipped more than $130,000 from the previous week (ringing in at $681,867), a difference attributable to Usher’s absence over the weekend.