Remove unused VIRTUAL_UNIVERSITIES setting

It looks like this was part of the original rollout of the microsites
feature back in December 2013, but was quickly removed in February
2014. These references have been orphaned ever since.
I can find no other references to it in the platform, save these.

The relevant platform code is pruned here:
https://github.com/edx/edx-platform/pull/18072

playbooks/roles/edxapp/defaults/main.yml

@@ -554,7 +554,6 @@ EDXAPP_CODE_JAIL_LIMITS:
   CPU: 1
   PROXY: 0
 
-EDXAPP_VIRTUAL_UNIVERSITIES: []
 # Set the number of workers explicitly for lms and cms
 # Should be set to
 # EDXAPP_WORKERS:
@@ -1213,7 +1212,6 @@ generic_env_config:  &edxapp_generic_env
   ORA2_FILE_PREFIX: "{{ EDXAPP_ORA2_FILE_PREFIX }}"
   FILE_UPLOAD_STORAGE_BUCKET_NAME: "{{ EDXAPP_FILE_UPLOAD_STORAGE_BUCKET_NAME }}"
   FILE_UPLOAD_STORAGE_PREFIX: "{{ EDXAPP_FILE_UPLOAD_STORAGE_PREFIX }}"
-  VIRTUAL_UNIVERSITIES: "{{ EDXAPP_VIRTUAL_UNIVERSITIES }}"
   REGISTRATION_EXTRA_FIELDS: "{{ EDXAPP_REGISTRATION_EXTRA_FIELDS }}"
   XBLOCK_SETTINGS: "{{ EDXAPP_XBLOCK_SETTINGS }}"
   EDXMKTG_USER_INFO_COOKIE_NAME: "{{ EDXAPP_EDXMKTG_USER_INFO_COOKIE_NAME }}"