Android - Hide buttons overlay (e.g. 5 second burst) on Google Camera
I recently had the same issue with my 6P, running 6.0.1 and the latest Google Camera version 3.1.021. It turned out to be a result of having the Screenwise Trends Panel app installed on all my phones. Enabling it in Accessibility Services causes the buttons to show up. From my searching there is no real solution other than to disable the Accessibility Service toggle for Screenwise. Then the buttons go away.
The buttons overlay (i.e. Z-, Z+, and 5 SECOND BURST) are part of Android's accessibility feature, TalkBack. Refer to Google Camera Help - Use TalkBack with Google Camera.
To hide the buttons, you need to turn off TalkBack feature on Android Settings - Accessibility - TalkBack - tap to toggle off.
Note: the buttons are integrated to the TalkBack feature, so there's no way to hide the buttons without turning TalkBack feature.
I too had this menu enabled today. Later I found that, I was playing around with accessibility settings today. I've (accidentally) enabled "Switch access", so I got this menu in camera app.
Now after disabling the Switch access, this menu was gone in camera.