Remove old footer code (#11836)

Requests to `/_/api/v2/footer_html`:

* 3k in last 30 days in community:
https://dash.cloudflare.com/76f653d51b8df4e9e956eb83a5ff42f4/readthedocs.io/analytics/traffic?path=%2F_%2Fapi%2Fv2%2Ffooter_html&time-window=43200
* 1k in the last 30 days in commercial:
https://dash.cloudflare.com/76f653d51b8df4e9e956eb83a5ff42f4/readthedocs-hosted.com/analytics/traffic?path=%2F_%2Fapi%2Fv2%2Ffooter_html&time-window=43200

The amount of requests is pretty low, so I guess is just spam or
similar. I checked those Host from where the request was performed and
those projects are using addons correctly and not hitting the old footer
endpoint.

The only project that still hits `footer_html` API is
https://dogapi.readthedocs.io/en/latest/. This is because it's pretty
old (last build on 2016) and our CF Worker is not removing old URL of
our js. We can add it to
https://github.com/readthedocs/common/blob/f5a5c301a3fdb4b03d6176a6980c999c219b14a2/packages/addons-inject/index.js#L23-L54
if we want, tho.

Closes readthedocs/ext-theme#464

readthedocs/api/v2/permissions.py

@@ -29,7 +29,7 @@ class IsAuthorizedToViewVersion(permissions.BasePermission):
     """
     Checks if the user from the request has permissions to see the version.
 
-    This permission class used in the FooterHTML and PageSearchAPIView views.
+    This permission class used in PageSearchAPIView views.
 
     .. note::
 

readthedocs/api/v2/proxied_urls.py

@@ -8,14 +8,13 @@
 from django.urls import path
 
 from readthedocs.analytics.proxied_api import AnalyticsView
-from readthedocs.api.v2.views.proxied import ProxiedEmbedAPI, ProxiedFooterHTML
+from readthedocs.api.v2.views.proxied import ProxiedEmbedAPI
 from readthedocs.search.api.v2.views import ProxiedPageSearchAPIView
 
-api_footer_urls = [
-    path("footer_html/", ProxiedFooterHTML.as_view(), name="footer_html"),
+api_proxied_urls = [
     path("search/", ProxiedPageSearchAPIView.as_view(), name="search_api"),
     path("embed/", ProxiedEmbedAPI.as_view(), name="embed_api"),
     path("analytics/", AnalyticsView.as_view(), name="analytics_api"),
 ]
 
-urlpatterns = api_footer_urls
+urlpatterns = api_proxied_urls