ipmi: Remove unused struct watcher_entry

After commit e86ee2d4("ipmi: Rework locking and shutdown for hot remove"),
no one use struct watcher_entry, so remove it.

Signed-off-by: Yuan Can <yuancan@huawei.com>
Message-Id: <20220927133814.98929-1-yuancan@huawei.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>