dashboard: drop known issue for linux-riscv64-unmatched
dashboard: drop known issue for linux-riscv64-unmatched

The root cause of #53379 is misconfiguration of stage0
that we can drop this known issue now.

Fixes golang/go#53379

Change-Id: Ic8c4738a9b9ab9f38504f0e54cc69137062c491a
Reviewed-on: https://go-review.googlesource.com/c/build/+/414934
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Meng Zhuo <3e6d300a356a3b86bc195677414a9ec5508ca893@golangcn.org>
Run-TryBot: Meng Zhuo <3e6d300a356a3b86bc195677414a9ec5508ca893@golangcn.org>
Auto-Submit: Meng Zhuo <3e6d300a356a3b86bc195677414a9ec5508ca893@golangcn.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

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