gnome-session: Permit per-box selection of gnome session

Some of my boxes want a dummy gnome session because they use systemd to mange
the user session. Other boxes want a vanilla gnome session.

However, the makefiles ignored this pre-box setup.

To permit each box to choose its own gnome session, remove
    $git_top/.config/gnome-session/session/gnome.session
    $git_top/.config/gnome-session/session/gnome-fallback.session
and fix Home.mk to not delete the corresponding real files in $HOME.