settings: Eliminate disused variable EXTERNAL_API_URI.

This was part of the logic to handle EXTERNAL_API_PATH varying.
But also it was already no longer used -- it was only ever passed
into template contexts, as `external_api_uri`, and it'd been
overtaken there by `external_api_uri_subdomain`.

So, update our dev docs to reflect that, and eliminate the variable.

docs/integration-docs-guide.md

@@ -99,7 +99,7 @@ Here are a few common macros used to document Zulip's integrations:
 
 * **Contents:** See [source][5]. **Note:** If special configuration is
   required to set up the URL and you can't use this macro, be sure to use the
-  `{{ external_api_uri }}` template variable, so that your integration
+  `{{ external_api_uri_subdomain }}` template variable, so that your integration
   documentation will provide the correct URL for whatever server it is
   deployed on.  If special configuration is required to set the SITE
   variable, you should document that too.

zerver/context_processors.py

@@ -111,7 +111,6 @@ def zulip_default_context(request):
         'privacy_policy': settings.PRIVACY_POLICY,
         'login_url': settings.HOME_NOT_LOGGED_IN,
         'only_sso': settings.ONLY_SSO,
-        'external_api_uri': settings.EXTERNAL_API_URI,
         'external_host': settings.EXTERNAL_HOST,
         'external_uri_scheme': settings.EXTERNAL_URI_SCHEME,
         'realm_invite_required': realm_invite_required,

zproject/settings.py

@@ -604,7 +604,6 @@ else:
 ########################################################################
 
 ROOT_DOMAIN_URI = EXTERNAL_URI_SCHEME + EXTERNAL_HOST
-EXTERNAL_API_URI = ROOT_DOMAIN_URI + "/api"
 
 if "NAGIOS_BOT_HOST" not in vars():
     NAGIOS_BOT_HOST = EXTERNAL_HOST