integration-docs: Remove create channel for generate webhook URL docs.

Updates text of generate-webhook-url-basic.md to replace the use of
the "Create the channel" step.

Removes {!create-channel.md!} from all the integrations docs that
also include {!generate-webhook-url-basic.md!}.

Fixes #32269.

zerver/webhooks/harbor/doc.md

@@ -6,8 +6,6 @@ Harbor's webhooks feature is available in version 1.9 and later.
 
 {start_tabs}
 
-1. {!create-channel.md!}
-
 1. {!create-an-incoming-webhook.md!}
 
 1. {!generate-webhook-url-basic.md!}