Removed TinyMCE NPM dep

We don't have an easy way to copy over those files automatically into the thirdparty/ dir,
where they are accessible on the webroot (not the case in node_modules).

To avoid confusion, we don't duplicate into node_modules for now.
See https://github.com/silverstripe/silverstripe-framework/pull/5918#issuecomment-244910280

package.json

@@ -59,7 +59,6 @@
     "redux": "https://registry.npmjs.org/redux/-/redux-3.0.5.tgz",
     "redux-thunk": "^2.1.0",
     "tether": "^1.3.2",
-    "tinymce": "^4.3.3",
     "url": "^0.11.0"
   },
   "devDependencies": {