I think it used to be possible, fairly certain i had removed it.
Not surprised that Gnome help doesn't have how to remove or add toolbar buttons or otherwise customize the icons on notes.
But i am surprised that https://help.gnome.org/users/tomboy/stable/index.html.en does not say anywhere how to get involved at all, how to contribute, nor how to ask for help or support.
Eventually found this:
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