do not show search scopes on main/repogroup search pages

Created by: sqs

Search scopes are still suggested as filters on the search results page, but they are no longer shown underneath the center search input on the main /search page or on repogroup search pages.

These were not very useful. The default ones on Sourcegraph.com were: "Test code", "Non-test files", "JSON files", "Text documents", "Non-vendor code", and "Vendor code". If a user clicked on these from the /search page, they would get a bunch of fairly arbitrary results; none were useful as the first step in a search. Usage was low.

Also, they conflicted with the much more useful repogroups and other search examples, which are much more effective at helping a user start their first search.

On self-hosted instances, the recommended replacement (mentioned in the CHANGELOG.md) is better because it lets you define useful links to show on the homepage without also showing those links as suggested filters on every search (where they are unlikely to be relevant).