Do not persist extensions gallery workspace variables if marketplace is disabled
MR: Pending
## Description
The following discussion from !147718+ should be addressed:
- [ ] https://gitlab.com/gitlab-org/gitlab/-/merge_requests/147718#note_1855490478:
> Regarding question above:
>
> > **question; follow-up; non-blocking**
> >
> > When `GL_EDITOR_ENABLE_MARKETPLACE` is false, `GL_EDITOR_EXTENSIONS_GALLERY_SERVICE_URL`, `GL_EDITOR_EXTENSIONS_GALLERY_ITEM_URL`, `GL_EDITOR_EXTENSIONS_GALLERY_RESOURCE_URL_TEMPLATE` are technically not required. They are just extra entries in the DB. Should we then add them to the workspaces_variables? Shouldn't we make it conditional?
>
> Yes, this is a good point. Lets make this a follow-up issue.
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