integration-docs: Update branch filtering instructions.

Updated the Git-related integrations that support branch filtering:
- Removed the branch-filtering configuration option, and its macro.
- Added a new macro that integrates the branch-filtering instruction
with the Generate-URL step.

The new macro `generate-webhook-url-with-branch-filtering` builds off of
the `generate-webhook-url-basic` macro, adding only the branch filtering
instruction. But, it does not re-use the macro, instead duplicates it,
to avoid the newline break that would be added otherwise.

zerver/webhooks/gitea/doc.md

@@ -6,7 +6,7 @@ Receive Gitea notifications in Zulip!
 
 1. {!create-an-incoming-webhook.md!}
 
-1. {!generate-webhook-url-basic.md!}
+1. {!generate-webhook-url-with-branch-filtering.md!}
 
 1. Go to your repository on Gitea and click on **Settings**. Select
    **Webhooks** on the left sidebar, and click **Add Webhook**.
@@ -22,10 +22,6 @@ Receive Gitea notifications in Zulip!
 
 ![](/static/images/integrations/gitea/001.png)
 
-### Configuration options
-
-{!git-branches-additional-feature.md!}
-
 ### Related documentation
 
 {!webhooks-url-specification.md!}