(core) Restoring GRIST_DEFAULT_PRODUCT functionality

Summary: The GRIST_DEFAULT_PRODUCT wasn't used for grist-ee, now it is respected.

Test Plan:
I've build grist-ee docker image from github and run it using our instruction (both for recreating the issue and confirming it is fixed)
```
docker run -p 8484:8484 \
  -v $PWD:/persist \
  -e GRIST_SESSION_SECRET=invent-a-secret-here \
  -e GRIST_SINGLE_ORG=cool-beans
  -it gristlabs/grist-ee
```

For grist-core I recreated/confirmed it is fixed it just by `GRIST_SINGLE_ORG=team npm start` in the core folder.
I also created some team sites using stubbed UI and confirmed that they were using the GRIST_DEFAULT_PRODUCT product.

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D4271

stubs/app/server/server.ts

@@ -7,7 +7,6 @@
 import {commonUrls} from 'app/common/gristUrls';
 import {isAffirmative} from 'app/common/gutil';
 import {HomeDBManager} from 'app/gen-server/lib/HomeDBManager';
-import {TEAM_FREE_PLAN} from 'app/common/Features';
 
 const debugging = isAffirmative(process.env.DEBUG) || isAffirmative(process.env.VERBOSE);
 
@@ -90,7 +89,6 @@ async function setupDb() {
       }, {
         setUserAsOwner: false,
         useNewPlan: true,
-        product: TEAM_FREE_PLAN
       }));
     }
   }