Remove section about modal feature flag that is no longer relevant (#925)

This is no longer hidden behind a feature flag

Author: pjlast

docs/admin/auth/index.mdx

@@ -533,9 +533,3 @@ Usernames from authentication providers are normalized before being used in Sour
 For example, a user whose external username (according the authentication provider) is `[email protected]` would have the Sourcegraph username `alice-smith`.
 
 If multiple accounts normalize into the same username, separate accounts are still created, but Sourcegraph will add a randomized suffix to the username to ensure uniqueness.
-
-## Remind users to connect external accounts
-
-When authorization is activated, users must link their external code host accounts with their Sourcegraph accounts to access private repositories.
-To ensure users are aware of any unlinked accounts, enable the external-accounts-modal feature flag.
-This prompts users to connect any missing external accounts upon visiting Sourcegraph.