dashboard: update Windows builders, remove old manual Windows builder
dashboard: update Windows builders, remove old manual Windows builder

And clean up some other builders.

Change-Id: I1f71836da7008e58d3e76e2cc3170e96cd57ddf6
Reviewed-on: https://go-review.googlesource.com/51432
Reviewed-by: Jeff Johnson <e0aeede46a59107c4d834f64851ee491686571e7@google.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build