Currently when there are no more auto follow patterns for a remote cluster then
the AutoFollower instance for this remote cluster will be removed. If
a new auto follow pattern for this remote cluster gets added quickly enough
after the last delete then there may be two AutoFollower instance running
for this remote cluster instead of one.

Each AutoFollower instance stops automatically after it sees in the
start() method that there are no more auto follow patterns for the
remote cluster it is tracking. However when an auto follow pattern
gets removed and then added back quickly enough then old AutoFollower
may never detect that at some point there were no auto follow patterns
for the remote cluster it is monitoring. The creation and removal of
an AutoFollower instance happens independently in the `updateAutoFollowers()`
as part of a cluster state update.

By adding the `removed` field, an AutoFollower instance will not miss the
fact there were no auto follow patterns at some point in time. The
`updateAutoFollowers()` method now marks an AutoFollower instance as
removed when it sees that there are no more patterns for a remote cluster.
The updateAutoFollowers() method can then safely start a new AutoFollower
instance.

Relates to elastic#36761

…d_flag