Drop Webhooks from project import/export config
Webhook URLs were recently encrypted in the database via
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21645, and as a
result they are no longer exported. When an exported project with
Webhooks is imported, the project import service will fail with
URI::InvalidURIError: bad URI(is not URI?) due to a blank URL.
We avoid this by disabling the export/import of Webhooks in
the first place.