Cleanup: remove pinning for xresprobe.

This package has not been installed in our Squeeze-based Debian Live images for
weeks.

config/chroot_apt/preferences

@@ -2,10 +2,6 @@ Package: aircrack-ng
 Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
-Package: xresprobe
-Pin: release o=Debian,a=unstable
-Pin-Priority: 999
-
 Explanation: weirdness in chroot_apt install-binary
 Package: *
 Pin: release o=chroot_local-packages