dashboard: remove nonexistent linux-mips builders
dashboard: remove nonexistent linux-mips builders

These builders haven't existed for a while, and they consume
horizontal space on the build dashboard, pushing more interesting
columns off of the screen at moderate resolutions.

Updates golang/go#31217

Change-Id: Ib7021b699956d01d17e0ea8b213efd07b17d26f7
Reviewed-on: https://go-review.googlesource.com/c/build/+/193017
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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