How to get rid of the “Do Not Disturb” Menu Bar icon?
Uncheck the Show in Menu Bar option located in System Preferences > Dock & Menu Bar > Do Not Disturb
Note that you can also show the icon only when Do Not Disturb is active by checking the option and select when active option.
@tama's answer is correct, but as a quicker way you can also hold the ⌘ Command key and drag the icon out of the menubar to remove it.
This trick works with a number of the macOS system icons in the menubar.