integrations-docs: Remove `zulip-config.md`.

The content in `zulip-config.md` was moved (in #22315) to
`change-zulip-config.file.md`. This removes the now out-of-date
reference to this macro in the documentation for writing
integrations and removes the file as it is no longer in use.

docs/documentation/integrations.md

@@ -130,9 +130,6 @@ Here are a few common macros used to document Zulip's integrations:
 - `{!webhook-url.md!}` - Used internally by `{!create-bot-construct-url.md!}`
   to generate the webhook URL.
 
-- `{!zulip-config.md!}` - Used internally by `{!change-zulip-config-file.md!}`
-  to specify the lines in the config file for a non-webhook integration.
-
 - `{!webhook-url-with-bot-email.md!}` - Used in certain non-webhook integrations
   to generate URLs of the form:
 

templates/zerver/help/include/zulip-config.md

@@ -1,5 +0,0 @@
-```
-ZULIP_USER = "{{ integration_name }}-bot@example.com"
-ZULIP_API_KEY = "0123456789abcdef0123456789abcdef"
-ZULIP_SITE = "{{ api_url }}"
-```