1. Drop overrides to force installing packages from Debian stable:  that's the
   default, and we don't need to override it since the devel APT suite has no
   package meant for Squeeze anymore.

2. Downgrade to 999 the pinning for packages we fetch from backports:  we don't
   need to set a higher value anymore, since the devel APT suite has no package
   meant for Squeeze.

3. Downgrade to 990 the pinning for I2P packages: no need to override
   Squeeze-specific packages anymore, so let's just use the same priority
   we assign to the regular Debian stable archive.