libweston: stop auto-adding the implicit head

All frontends have been converted to the new head-based output
management API, which means that
weston_compositor_create_output_with_head() is calling
weston_output_attach_head(). We will never hit the implicit attach
anymore.

Therefore we can now require that an output has at least one head when
calling weston_output_enable(). An output without heads is useless.

The auto-add code is removed as dead.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>