DM WebUI need to remove dm-worker offline option

[XuJianxu]

What did you do?

• deploy dm cluster，create source
• webui-> Migration->Cluster->members->workers, try to click offline (not work)

I suggest to remove this click on the WebUi cause it's not the correct function.

What did you expect to see?

worker can offline

What did you see instead?

Versions of the cluster

DM version (run dmctl -V or dm-worker -V or dm-master -V):

nightly

Upstream MySQL/MariaDB server version:

(paste upstream MySQL/MariaDB server version here)

Downstream TiDB cluster version (execute SELECT tidb_version(); in a MySQL client):

(paste TiDB cluster version here)

How did you deploy DM: tiup or manually?

(leave TiUP or manually here)

Other interesting information (system version, hardware config, etc):

>
>

current status of DM cluster (execute query-status <task-name> in dmctl)

(paste current status of DM cluster here)