Cleanup APT preferences now that feature/wheezy was merged into devel.
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.