[sync/dispatcher] Add TODO to remove context from Channel.
[sync/dispatcher] Add TODO to remove context from Channel.

Removing ctx will clarify Channel's behavior; if users want to attach
closure of the Channel to a context from their writing goroutine, they
are free to do `defer close(channel.C)` in their main write function.

R=tandrii@chromium.org

Change-Id: Ifd48ca14d48f3d7b9bbd5b9f2d95c18de3232997
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1809487
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go