fix: remove unused subprocess import, guard logs dir in start_tunnel

tests/unit/test_process.py

@@ -109,7 +109,6 @@ def test_read_pid_file_corrupt(config_dir):
 
 import shutil
 import signal
-import subprocess
 import time
 from autossh_mgr.config import TunnelConfig
 from autossh_mgr.process import start_tunnel, stop_tunnel, build_autossh_cmd