tempo-mixin: clear allValue so selecting All namespaces only queries Tempo namespaces

[yvrhdn]

What this PR does:

This is a temporary patch until grafana/jsonnet-libs#703 is merged. Update the grafana-builder jsonnet library to include grafana/jsonnet-libs#703
More details can be found in that issue as well.

This dropdown is populated with the namespaces a Tempo cluster is running in. The current behaviour (with allValue: '.+') is confusing, because selecting All in the dropdown ends up querying all namespaces.
The previous behaviour (with allValue: null) only queries the namespaces that are returned by the query.

The reason we got confused is that in our dashboard you can select a cluster and if you know there is only 1 Tempo namespace in that cluster you kind of expect to only see data from that namespace. But with allValue: '.+' you get data from all the namespaces in that cluster. So we were mistakenly looking at queriers from a non-Tempo cluster and wondering why CPU was so high.