chore: remove SITE_ID middleware

I do not think it is a useful solution for TUP-706.

The code/feature is archived in #873:
#873

Author: wesleyboar

taccsite_cms/custom_app_settings.example.py

@@ -1,3 +1,3 @@
 CUSTOM_APPS = ['apps.custom_example', 'apps.djangocms_blog_customizations']
-CUSTOM_MIDDLEWARE = ['taccsite_cms.middleware.settings.DynamicSiteIdMiddleware']
+CUSTOM_MIDDLEWARE = []
 STATICFILES_DIRS = ()