ref(builder): Remove /buildpacks override

The /buildpacks override in deis-builder never worked in the first
place. When removed, this reduces an extra edge case to test for
when testing deis-builder. In addition, there are multiple ways of
setting your own custom buildpacks:
* custom slugbuilder (requires custom deis-builder)
* multi-buildpack (.buildpacks file)
* BUILDPACK_URL environment variable

Author: Arne-Christian Blystad

builder/image/templates/builder

@@ -94,12 +94,6 @@ BUILD_OPTS+=$(echo $RESPONSE | /app/bin/get-app-values)
 # if no Dockerfile is present, use slugbuilder to compile a heroku slug
 # and write out a Dockerfile to use that slug
 if [ ! -f Dockerfile ]; then
-    if [ -f /buildpacks ]; then
-        BUILD_OPTS+=' -v /buildpacks:/tmp/buildpacks:rw'
-        # give non-root slugbuilder user R/W perms for docker volumes
-        chown -R 2000:2000 /buildpacks
-    fi
-
     # run in the background, we'll attach to it to retrieve logs
     BUILD_OPTS+=' -d'
     BUILD_OPTS+=' -v '