remove unused method setDuration()

DcCore/DcCore/DC/Wrapper.swift

@@ -1014,10 +1014,6 @@ public class DcMsg {
     public func setDimension(width: CGFloat, height: CGFloat) {
         dc_msg_set_dimension(messagePointer, Int32(width), Int32(height))
     }
-    
-    public func setDuration(_ duration: Double) {
-        dc_msg_set_duration(messagePointer, Int32(duration))
-    }
 
     public var filesize: Int {
         return Int(dc_msg_get_filebytes(messagePointer))