backend: plugins: Remove .plugins from watch list

[skoeva]

This fix prevents the development app (when running npm run dev-only-app) from watching for static plugins locally, which causes an endless loop of refreshes to the app.

Fixes: #1937

Testing

• [x] Create .plugins folder locally (e.g. by running make app-linux)
• [x] Run make frontend, make backend and npm run dev-only-app
• [x] Ensure that "Plugins are loaded" message is only outputted once by the app
• [x] Ensure that the backend does not send "Sending reload plugins signal to frontend" in a loop