remove get-gluon-branch.sh

It is no longer used, so remove.

get-gluon-branch.sh

-#!/bin/bash
-
-set -eu
-set -o pipefail
-
-# autoupdater
-# ====
-#
-# This script prints the name of the autoupdater branch depending on the build
-# configuration. Its output is suitable for use as the GLUON_BRANCH build
-# variable of gluon.
-#
-# The idea is to take whatever comes in the GLUON_BRANCH build variable,
-# except when this value is not one of the well-known branch names; then fall
-# back to 'nightly'.
-
-default_gluon_branch_name="nightly"
-
-case "${GLUON_BRANCH:-}" in
-    "nightly" | "beta" | "stable")
-        gluon_branch="$GLUON_BRANCH"
-        ;;
-    *):
-        gluon_branch="$default_gluon_branch_name"
-        ;;
-esac
-
-echo $gluon_branch