remove js packages that are not referenced in js code (#5514)

superset/assets/package.json

@@ -51,7 +51,6 @@
     "bootstrap": "^3.3.6",
     "bootstrap-slider": "^10.0.0",
     "brace": "^0.11.1",
-    "brfs": "^1.4.3",
     "classnames": "^2.2.5",
     "d3": "^3.5.17",
     "d3-cloud": "^1.2.1",
@@ -79,13 +78,8 @@
     "moment": "^2.20.1",
     "mousetrap": "^1.6.1",
     "mustache": "^2.2.1",
-    "npm": "^6.1.0",
     "nvd3": "1.8.6",
-    "object.entries": "^1.0.4",
-    "object.keys": "^0.1.0",
-    "object.values": "^1.0.4",
     "parse-iso-duration": "^1.0.0",
-    "po2json": "^0.4.5",
     "prop-types": "^15.6.0",
     "re-resizable": "^4.3.1",
     "react": "^15.6.2",
@@ -164,6 +158,7 @@
     "mini-css-extract-plugin": "^0.4.0",
     "mocha": "^3.2.0",
     "npm-check-updates": "^2.14.0",
+    "po2json": "^0.4.5",
     "prettier": "^1.12.1",
     "react-addons-test-utils": "^15.6.2",
     "react-test-renderer": "^15.6.2",