main.star: remove known issue for linux-s390x builder

The builder has been added and it's operational.

Fixes golang/go#67307.

Change-Id: I11cf987bc8a7d6c9f9cb9fa9dde4950b196c4397
Reviewed-on: https://go-review.googlesource.com/c/build/+/671959
Commit-Queue: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: dmitshur