bus: Don't explicitly clear BusConnections.monitors

bus: Don't explicitly clear BusConnections.monitors

Each connection that is an active monitor holds a pointer to its own
link in this list, via BusConnectionData.link_in_monitors. We can't
validly free the list while these pointers exist: that would be a
use-after-free, when each connection gets disconnected and tries to
remove itself from the list.

Instead, let each connection remove itself from the list, then assert
that the list has become empty.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Resolves: https://gitlab.freedesktop.org/dbus/dbus/issues/291

(cherry picked from commit b034b83b59efffd4cc819ad42d0cd078d91d53df)