You can remove it by setting "Touch UI" to off in the Options window ingame.
This is on by default, so the player needs to turn it off whenever they launch your game for the first time.
Also, even if you turn off the button, there will still be a gap between the menu and the upper edge of the screen, which is weird and ugly.
I've made a plugin that improves the touch UI button:
https://forums.rpgmakerweb.com/index.php?threads/n_enhancedtouchuibutton.127009/
With that plugin, you can:
- Permanently enable or disable the button. The button is permanently enabled on mobiles.
- Get rid of that gap when the button is turned off.
- Let the player move the button to the bottom, if they so wish.
- Choose whether the button should be turned on or off by default (when the game is launched for the first time).
PS: I just noticed a bug that only allows players to move the button to the bottom if they're not on mobile.
Once I've fixed it, I'll release a version 1.0.1. It's fixed now.