This is actually an annoying problem in Gimp; I often find myself having projects with multiple layers, but with a bottom layer without transparency, so having an alpha channel on the final exported image is a complete waste of space on that. Gimp doesn't seem to detect that final lack of transparency, though, so when I export to png, it always ends up containing an alpha channel.
The best solution for this seems to be to either use the "Flatten Image" function before exporting (available in the right-click menu of the Layers dialog, or just in the menus under Image), or to just export the image, then open the exported file (it'll be easily accessible in the "Open Recent" menu), and then removing the now-single layer's alpha channel and using the function to export to the same filename again.
When working with Gimp xcf projects I generally prefer the second method, since it means there is no risk of accidentally saving your flattened project and losing all your layers.
In newer Gimp versions, the save options popup for png format has a list of pixel formats, allowing you to select "8bpc RGB" to save without alpha channel. However, be careful when using this, because the application will remember this choice for other pngs saved within the same run of the program. So if you have a png with transparency you want to save after that, you have to remember to specifically change the option back to "automatic" or to the specific "8bpc rgba".