remove unused isIoRunning() api

the api is about to be removed from core as well,
see https://github.com/deltachat/deltachat-core-rust/pull/2139

DcCore/DcCore/DC/Wrapper.swift

@@ -224,10 +224,6 @@ public class DcContext {
         dc_stop_io(contextPointer)
     }
 
-    public func isIoRunning() -> Bool {
-        return dc_is_io_running(contextPointer) != 0
-    }
-
     public func setStockTranslation(id: Int32, localizationKey: String) {
         dc_set_stock_translation(contextPointer, UInt32(id), String.localized(localizationKey))
     }