Revert "fix(builder): strip single quotes from BUILDPACK_URL"

This reverts commit 1598d72.

Author: Matthew Fisher

builder/image/slugbuilder/builder/build.sh

@@ -77,9 +77,6 @@ selected_buildpack=
 if [[ -n "$BUILDPACK_URL" ]]; then
     echo_title "Fetching custom buildpack"
 
-    # FIXME: strip single quotes coming from the builder
-    BUILDPACK_URL=$(echo $BUILDPACK_URL | tr -d "'")
-
     buildpack="$buildpack_root/custom"
     rm -fr "$buildpack"