aggregateMenu: Don't use system menu for width computation

aggregateMenu: Don't use system menu for width computation

If the user's real name is too long to fit the menu comfortably, we are
supposed to use the username instead. However since commit f8e5e3e435f,
we no longer set a max-width on the menu as a whole, but instead base
the width request on only "unellipsizable" children. For some reason
the system menu ended up there, so the name is now allowed to grow
indefinitely.

Remove it from the list of size children to get the intended behavior
back.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/400