Alert tells user to enable repositories even though the functionality has been removed

Created by: mrnugget

Sourcegraph version: 3.4

Now that we have removed the user-visible parts of the Enabled and Disabled flags in https://github.com/sourcegraph/sourcegraph/pull/3998 and https://github.com/sourcegraph/sourcegraph/pull/3970 we should probably change the wording of this alert:

Screen Shot 2019-05-20 at 16 05 47

This is the NoRepositoriesEnabledAlert defined here: https://sourcegraph.sgdev.org/sourcegraph/sourcegraph@25226c3/-/blob/web/src/site/NoRepositoriesEnabledAlert.tsx

It gets displayed depending on the value of the global noRepositoriesEnabled field: https://sourcegraph.sgdev.org/search?q=repo:%5Esourcegraph/sourcegraph%24%4025226c3+noRepositoriesEnabled