clean up dynamic plugin loading

Summary:
There are 3 sources where plugins can be loaded from:
* `src/plugins`
* `src/fb/plugins`
* any path specified in `~/.sonar/config.json`

Plugins found in the first two directories are bundled with the app when building.

Reviewed By: jknoxville

Differential Revision: D8636061

fbshipit-source-id: 2064090d43d11695ffd99df195e5b594559fe087

scripts/start-dev-server.js

@@ -26,7 +26,6 @@ function launchElectron({bundleURL, electronURL}) {
   const args = [
     path.join(STATIC_DIR, 'index.js'),
     '--remote-debugging-port=9222',
-    '--dynamicPlugins=~/fbsource/xplat/sonar/src/plugins,~/fbsource/xplat/sonar/src/fb/plugins',
   ];
 
   const proc = child.spawn(electronBinary, args, {