fix: remove probably unnecessary unregister

When the refresh button is clicked in the navigation drawer, then
startPushService is executed which registers the handler. The
WebSocketService itself is reused, thus isn't created again or
destroyed. From my tests the callback isn't registered twice after the
refresh. Thus, I'd expect the unregister isn't needed at all because
android somehow does this automagically.

app/src/main/kotlin/com/github/gotify/service/WebSocketService.kt

@@ -5,7 +5,6 @@ import android.app.Notification
 import android.app.NotificationManager
 import android.app.PendingIntent
 import android.app.Service
-import android.content.Context
 import android.content.Intent
 import android.graphics.Color
 import android.net.ConnectivityManager
@@ -79,10 +78,6 @@ internal class WebSocketService : Service() {
     override fun onDestroy() {
         super.onDestroy()
         connection?.close()
-        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
-            (getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager)
-                .unregisterNetworkCallback(networkCallback)
-        }
 
         Log.w("Destroy ${javaClass.simpleName}")
     }