Fix Errno::ECONNRESET error for Tubesock

* We must not close the socket manually (with `client_socket.close`), as this would close it twice. Compare: 2c83794b85/lib/tubesock.rb (L92)
* When the socket is closed twice, nginx registers a `Connection reset by peer` error: `recv() failed (104: Connection reset by peer) while proxying upgraded connection`
* Tubesock automatically closes the socket when the `hijack` block ends and otherwise ignores `Errno::ECONNRESET`: 2c83794b85/lib/tubesock.rb (L161-L162)

app/controllers/submissions_controller.rb

@@ -313,7 +313,9 @@ class SubmissionsController < ApplicationController
     # We don't want to store this (arbitrary) exit command and redirect it ourselves
     client_socket.send_data JSON.dump({cmd: :exit})
     client_socket.send_data nil, :close
-    client_socket.close
+    # We must not close the socket manually (with `client_socket.close`), as this would close it twice.
+    # When the socket is closed twice, nginx registers a `Connection reset by peer` error.
+    # Tubesock automatically closes the socket when the `hijack` block ends and otherwise ignores `Errno::ECONNRESET`.
   end
 
   def create_remote_evaluation_mapping