main.star: remove known issue for openbsd-riscv64 luci builder

This builder has been migrated and appears to be running stably.

Updates golang/go#67105

Change-Id: Iaafaf669a290b5fc1fda2deea1ed06b2faf7ec60
Reviewed-on: https://go-review.googlesource.com/c/build/+/598155
Reviewed-by: Cherry Mui <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: 4a6f656c