DashboardBuilders, SlowBots: remove documentation for legacy builders

The legacy builders have been fully stopped and don't work anymore.
The documentation for them is obsolete and misleading. Remove it.

For golang/go#67761.

Change-Id: I71d3568d7dfee2cd24012c4a03dd955cc2df8359
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/796382
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>