chore: remove unused gulp helpers (#9651)

* chore: remove unused dev dependency gulp-insert

* chore: remove unused dev dependency gulp-series

* chore: remove unused dev dependency gulp-shell

Author: rachel-fenichel

package-lock.json

@@ -126,11 +126,8 @@
     "gulp-concat": "^2.6.1",
     "gulp-gzip": "^1.4.2",
     "gulp-header": "^2.0.9",
-    "gulp-insert": "^0.5.0",
     "gulp-rename": "^2.0.0",
     "gulp-replace": "^1.0.0",
-    "gulp-series": "^1.0.2",
-    "gulp-shell": "^0.8.0",
     "gulp-sourcemaps": "^3.0.0",
     "gulp-umd": "^2.0.0",
     "http-server": "^14.0.0",