[thanos] - remove thanos-staging gstg and pre sidecars

What

• removes the sidecars for gstg and pre from thanos-staging queriers.

Why

gstg and pre now have a remote write prometheus instance deployed, and this removes the need to query the instances directly.
As this is currently experimental, this is just applied to the staging thanos.

Related Issue: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/24515