fix: remove incorrect Fleet auth callout (#4191)

## Description
Keeps the Fleet docs PAT and `X-Auth-Scheme` guidance while removing
only the incorrect custom-auth token sentence.

## Release Note
none

## Test Plan
- [ ] Preview the Fleet code docs page and confirm the auth callout only
mentions PAT and `X-Auth-Scheme`.

_Opened collaboratively by Brace Sproul and open-swe._

---------

Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>
Co-authored-by: Brace Sproul <46789226+bracesproul@users.noreply.github.com>

Author: 3 people

src/langsmith/fleet/code.mdx

@@ -122,7 +122,7 @@ curl --request GET \
 </Tabs>
 
 <Callout icon="key" color="#FEF3C7" iconType="regular">
-    Use a [Personal Access Token (PAT)](/langsmith/create-account-api-key) tied to your LangSmith account. Set the `X-Auth-Scheme` header to `langsmith-api-key` for authentication. If you implemented custom authentication, pass the user's token in headers so the agent can use user-scoped tools. See [Add custom authentication](/langsmith/custom-auth).
+    Use a [Personal Access Token (PAT)](/langsmith/create-account-api-key) tied to your LangSmith account. Set the `X-Auth-Scheme` header to `langsmith-api-key` for authentication.
 </Callout>
 
 ## 3. Invoke agent