xsfbs.mk: remove more obsolete code
xsfbs.mk: remove more obsolete code

We can use plain debconf-updatepo, and don't need to clean the 'imports'
directory or shlibs.local.

facboy/xserver-xorg-video-qxl,philipn/xserver-xorg-input-synaptics,RsrchBoy/xserver-xorg-input-synaptics,facboy/xserver-xorg-video-qxl,facboy/xserver-xorg-video-qxl,facboy/xserver-xorg-video-qxl,philipn/xserver-xorg-input-synaptics,philipn/xserver-xorg-input-synaptics,RsrchBoy/xserver-xorg-input-synaptics