Remove remnants of Gulp pipeline

We use Django Pipeline now, no need for Gulp
DemocracyClub · boffbowsh · Oct 5, 2019 · Jul 14, 2019 · Jul 21, 2019 · Jul 21, 2019
commit a1383966edf9b41b7574c55eacb034b72bc1055a
diff --git a/.bowerrc b/.bowerrc
diff --git a/bower.json b/bower.json
diff --git a/electionleaflets/ember.json b/electionleaflets/ember.json
diff --git a/ember.json b/ember.json
diff --git a/gulpfile.js b/gulpfile.js