bootstrap: do not pull in xbase-clients by default

bootstrap: do not pull in xbase-clients by default

It depends on x11-apps, x11-session-utils, x11-utils, x11-xkb-utils,
x11-xserver-utils, xauth, xinit which in turn depend on mesa/glx/drm
libraries, all in all accumulating to ~200 MiB of disk space for
stuff that most container applicaitons nowadays won't have any use.

Note that the original reasoning was probably `xauth` for X11 display
redirection, but for that there's a package with the same name that
has a much smaller disk usage footprint overall.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>