web: Remove NoRepositoriesEnabledAlert and noRepositoriesEnabled flag

Created by: mrnugget

This fixes #4127 by

1. removing the NoRepositoriesEnabledAlert
2. deprecating the noRepositoriesEnabled SiteFlag and always have it be false
3. removing every usage of the noRepositoriesEnabled SiteFlag in the web app

This should be reviewed by someone who knows the web component better than I do and is able to check whether I missed something when removing usage of the field or by deprecating the field.

Test plan: I manually

1. setup a fresh Sourcegraph instance, added an external service config that produces a repository
2. setup a fresh Sourcegraph instance, added an external service config that produces no repository

Created by: mrnugget

Added the release-blocker label because @sqs said in Slack that this should be removed for 3.4

Created by: codecov[bot]

Codecov Report

Merging #4128 into master will increase coverage by 0.02%. The diff coverage is 0%.

Impacted Files	Coverage Δ
cmd/frontend/graphqlbackend/search_alert.go	11.29% <0%> (+0.43%)
cmd/frontend/graphqlbackend/site_flags.go	0% <0%> (ø)