Revert "Add a new (partial) dconf profile: "turn_off_xrandrplugin"."

This reverts commit a07971aa448e0de087b530f61e432351ebaa1a7c.

We do not need to turn the xrandr plugin off even conditionally
with the latest code.
Precise_Pangolin_by_Vlad_Gerasimov.jpg is too dark, the hostname
does not show clearly on the top panel, so use another picture
as the default lightdm background on Precise.