Cleanup Squeeze-specific bits from our infrastructure

Originally created by @intrigeri on #7215 (Redmine)

Once Tails 1.1 is out, we can get rid of quite some Squeeze-specific configs, chroots, etc. on our infrastructure:

• debomatic-squeeze-backports-mozilla APT suite
• squeeze-backports-mozilla debomatic chroot
• Git grep will tell you more

Related issues

• Blocked by tails/tails#6015