How to remove background in sketch?

Sketch 3 does have some bitmap editing options, but they are very limited.

Here’s more about Bitmap editing in Sketch.

To start editing an image, select it on the canvas and double-click. You will see the inspector update with a few special tools.

You might wanna try your luck with Magic wand and then the Vectorize option below it. Although you’d probably be better off redrawing it with the Vector tool.

I would recommend you use another application to edit bitmap images. One relatively cheap, option would be Pixelmator especially considering how much Sketch 3 costs. Then again you could do it in Gimp just the same.