Bootstrap 3, how disable auto-added Glyphicon?
I haven't tried it for Drupal 8, but its works for drupal 7. It should work for drupal 8 as well.

In your bootstrap theme setting, under bootstrap setting there is tab of General setting. Inside that tab the are settings for different bootstrap components. You will see Buttons there. Just open that section and un-check Iconize Buttons option. That's it.

This will disable all the auto icons in buttons.