You should customize the value of
mouse-buffer-menu-mode-mult, for example putting something like this in your init file (
(setq mouse-buffer-menu-mode-mult 0)
From the documentation:
mouse-buffer-menu-mode-mult is a variable defined in `mouse.el'.
Its value is 4
Group the buffers by the major mode groups on <C-down-mouse-1>?
This number which determines (in a hairy way) whether <C-down-mouse-1>
will split the buffer menu by the major modes (see
`mouse-buffer-menu-mode-groups') or just by menu length.
Set to 1 (or even 0!) if you want to group by major mode always, and to
a large number if you prefer a mixed multitude. The default is 4.
You can customize this variable.
This variable was introduced, or its default value was changed, in
version 20.3 of Emacs.