dashboard: remove stale machineType entries from builders

CL 413420 changed the default GCE instance type
from e2-highcpu-2 (2 vCPU, 2 GB) to e2-standard-8 (8 vCPU, 32 GB),
and the default containerized instance type
from e2-standard-4 (4 vCPU, 16 GB) to e2-standard-16 (16 vCPU, 64 GB).

Any entries in the dashboard table that previously opted in to bigger,
faster machines are now opting into smaller, slower machines.
That seems undesirable, so delete all the opt-in smaller, slower
machineType entries.

For golang/go#17104.

Change-Id: I88806c1586b219257229c9e2302464efdcc558a6
Reviewed-on: https://go-review.googlesource.com/c/build/+/419077
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Auto-Submit: Russ Cox <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Russ Cox <[email protected]>