column_family: Don't bother closing the flush_queue on stop()
column_family: Don't bother closing the flush_queue on stop()

When stopping a column family we issue a flush(), for which we wait.
Since writes are supposed to have stopped coming in, and also new
flush requests, there's no need to call and wait for the flush_queue
to be closed.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

avikivity/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla