main.star: remove linux-loong64 known issue

The builder is up and working. There are occasional temporary network
errors, but that can be improved upon separately.

Fixes golang/go#65398.

Change-Id: I91e9b1e96efd852b5394bd670dd52c421440e719
Reviewed-on: https://go-review.googlesource.com/c/build/+/584776
Auto-Submit: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: dmitshur