v3 docs: Remove download link from docker droplet

Docker droplets cannot be downloaded, so the docs should not show a
download link on the example of one.

[finishes #173184530]

Authored-by: Reid Mitchell <rmitchell@pivotal.io>

Author: reidmit

docs/v3/source/includes/api_resources/_droplets.erb

@@ -389,9 +389,6 @@
         "assign_current_droplet": {
           "href": "https://api.example.org/v3/apps/7b34f1cf-7e73-428a-bb5a-8a17a8058396/relationships/current_droplet",
           "method": "PATCH"
-        },
-        "download": {
-          "href": "https://api.example.org/v3/droplets/585bc3c1-3743-497d-88b0-403ad6b56d16/download"
         }
       },
       "metadata": {