search: Fix memory usage of unfiltered zoekt List calls

Created by: tsenart

After we rolled out https://github.com/sourcegraph/sourcegraph/commit/cfaf0165e7fc3e40ff6bd202a7cf1b53d501d934, there are two places that are called rather frequently that use the non minimal zoekt listing which now includes large language maps which was causing frontends in prod to OOM. This is compounded by (ah!) compound shards, since any repos in them would return the aggregate compound shard language map.

These are the call sites:

• https://sourcegraph.com/github.com/sourcegraph/sourcegraph@20877303dde8145bf2eea5679[…]1/-/blob/internal/usagestats/repositories.go?L53&subtree=true
• https://sourcegraph.com/github.com/sourcegraph/sourcegraph@20877303dde8145bf2eea5679[…]/internal/search/zoekt/indexed_search.go?L467:31&subtree=true