Remove broken su fallback.

Was broken by passing environment variable PYTHONPATH to process. Will
fix this if there is a use case for it.

sshuttle/client.py

@@ -298,7 +298,6 @@ class FirewallClient:
             argvbase += ['--syslog']
         argv_tries = [
             ['sudo', '-p', '[local sudo] Password: '] + argvbase,
-            ['su', '-c', ' '.join(argvbase)],
             argvbase
         ]