Setup: back out use of server-side last-channel data because it is stale
Setup: back out use of server-side last-channel data because it is stale

The server preference for last channel is apparently stale and unused
these days, and the web client uses browser storage to keep this
context. As a result we can't rely on this data from the server.

For more info, see:
https://github.com/matterhorn-chat/matterhorn/issues/283

matterhorn-chat/matterhorn,aisamanra/matterhorn