Facing criticism from a health workers union, Harvard Medical Faculty Physicians has decided to drop its late-night surcharge for emergency room patients at five Massachusetts hospitals. The doctors group said that it would no longer add $30 to bills for emergency care delivered between 10 p.m. and 8 a.m. The fee was attacked by a healthcare union that is trying to organize workers at Beth Israel Deaconess Medical Center, with which the doctors are affiliated.