check_postgres_backup: Don't try to use a virtualenv.

The dependencies for this tool are installed using apt.

puppet/zulip/files/nagios_plugins/zulip_postgres_common/check_postgres_backup

@@ -5,9 +5,6 @@ import subprocess
 from datetime import datetime, timedelta
 
 import sys
-sys.path.append('/home/zulip/deployments/current')
-import scripts.lib.setup_path_on_import
-
 import dateutil.parser
 import pytz