dashboard: remove linux-arm as a trybot
dashboard: remove linux-arm as a trybot

Fixes golang/go#22592
Updates golang/go#22749
Updates golang/go#22748

Change-Id: I6b58f21ececa212a12af8ef21cdef9c90515b837
Reviewed-on: https://go-review.googlesource.com/78032
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

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