Revert "Enable Wheezy proposed-updates APT repository."

This reverts commit dc3952867939bdfb55eaa251a6b2fc97fa0b3418.

The proposed updates are now in Wheezy.
Revert "Setup APT pinning to install packages from stable-proposed-updates."

This reverts commit 2982197209a19e768a32454236fdd29afdd4be32.

The proposed updates are now in Wheezy.
Merge branch 'feature/Debian-proposed-updates' into stable
Install syslinux-utils, to get isohybrid back (Closes: #8155).
Merge remote-tracking branch 'origin/bugfix/8155-get-isohybrid-back' into stable