Drop code to handle buildd images for <disco

We'll deal with those in stable updates instead.

debian/changelog

@@ -1,6 +1,8 @@
 livecd-rootfs (2.560) UNRELEASED; urgency=medium
 
   * Disable merged /usr in buildd images.
+  * Drop code to handle building buildd images for series earlier than
+    disco; we'll deal with those in stable updates instead.
 
  -- Colin Watson <cjwatson@ubuntu.com>  Fri, 01 Feb 2019 18:01:56 +0000
 

live-build/auto/config

@@ -859,35 +859,8 @@ case $SUBPROJECT in
 		add_package install policyrcd-script-zg2
 		add_package install pkgbinarymangler
 		add_package install ca-certificates
-		case $SUITE in
+		add_package install gpg
-			precise|trusty|xenial)
+		add_package install gpg-agent
-				;;
-			*)
-				# only needed in >= bionic
-				add_package install gpg
-				add_package install gpg-agent
-				;;
-		esac
-		case $SUITE in
-			precise|trusty|xenial)
-				# no longer needed in >= artful
-				add_package install pkg-create-dbgsym
-				# no longer needed in >= bionic
-				add_package install apt-transport-https
-				# no longer needed in >= cosmic
-				add_package install tzdata
-				;;
-			artful)
-				# no longer needed in >= bionic
-				add_package install apt-transport-https
-				# no longer needed in >= cosmic
-				add_package install tzdata
-				;;
-			bionic)
-				# no longer needed in >= cosmic
-				add_package install tzdata
-				;;
-		esac
 		add_package install fakeroot
 		add_package install build-essential
 		# Needed for LXD-based builds.