Unpin backports priority for buildd images.

There was a time historically where Launchpad buildd might have relied
on that behaviour, but this shouldn't be the case anymore as it sets
priority manually when building backports.
Meanwhile any other builds using buildd images (e.g. snapcraft)
shouldn't default to backports unless required. (lp: #2009871)

Refs:
- [1] https://git.launchpad.net/launchpad-buildd/commit?id=c2ebcb6752

live-build/buildd/includes.chroot/etc/apt/preferences.d/backports

@@ -1,3 +0,0 @@
-Package: *
-Pin: release a=*-backports
-Pin-Priority: 500