don't close connection in management command

[olivierdalang]

This removes close_old_connections() at the end of the management command, because this prevents using call_command in the context of tests :

from django.core.management import call_command
from django.test import TestCase

class MyTestCase(TestCase):
    def test_cron(self):
        ...
        call_command('runcrons')
        # test something else here
        ...

This fails with psycopg2.InterfaceError: connection already closed, as tests are run in transaction and django needs to rollback the transaction at the end of the test.

The close_old_connections() line was added in f7de420 back in 2013, I'm pretty sure in the mean time it's become useless.