sshcon: remove use of self.ssh_conf

[glicht]

Use of self.ssh_conf was left over after being removed at the following PR: #2846

This PR removes the use of it and uses host_config instead.

Encountered this when trying to use an ssh based DOCKER_HOST which uses a ProxyCommand config. Was getting the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/glicht/.pyenv/versions/3.10.1/lib/python3.10/site-packages/docker/client.py", line 45, in __init__
    self.api = APIClient(*args, **kwargs)
  File "/Users/glicht/.pyenv/versions/3.10.1/lib/python3.10/site-packages/docker/api/client.py", line 171, in __init__
    self._custom_adapter = SSHHTTPAdapter(
  File "/Users/glicht/.pyenv/versions/3.10.1/lib/python3.10/site-packages/docker/transport/sshconn.py", line 176, in __init__
    self._create_paramiko_client(base_url)
  File "/Users/glicht/.pyenv/versions/3.10.1/lib/python3.10/site-packages/docker/transport/sshconn.py", line 207, in _create_paramiko_client
    self.ssh_conf['proxycommand']
AttributeError: 'SSHHTTPAdapter' object has no attribute 'ssh_conf'