remove redundant signals

(uvicorn is doing it already)

examples/docker_image/app/main.py

@@ -1,11 +1,7 @@
 import os
-import signal
 
 from nicegui import app, ui
 
-signal.signal(signal.SIGINT, app.shutdown)
-signal.signal(signal.SIGTERM, app.shutdown)
-
 
 @ui.page('/')
 def index():