According to the manual you can't select what the code completion shows and not, since the code completion shows all the assets that it has to the apps disposal in its surrounding context. If xcode thinks it should first provide you with images from the asset catalogue, it will unfortunately do so.
You can turn the code completion of in the text editing preferences or you can toggle the code completion on and off when typing with Control-Space bar and you can dismiss the code completion with the Escape key. If you turned the code completion off in the text editing preferences you can still turn them on with Control-Space bar.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…