librbd/migration/HttpClient: drop SslHttpSession::m_ssl_enabled

librbd/migration/HttpClient: drop SslHttpSession::m_ssl_enabled

The remaining callers of disconnect() call it only when m_ssl_enabled
is set to true (i.e. after the handshake is completed):

- shut_down(), in STATE_READY
- maybe_finalize_reset(), very shortly after transitioning out of
  STATE_READY as part of performing a reset
- advance_state(), on a transition to STATE_READY that is intercepted
  by a previously delayed shut down

m_ssl_enabled isn't used outside of disconnect() and on top of that
is never cleared.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 8566224e9406abca42925f8045077141c2724bed)