Remove global register_schemes calls

Modifying the global urllib.parse/urlparse uses_* variables isn't needed anymore. Most aren't used and are only left for backwards_compatibility.
Only one that could be relevant would be `uses_param` (for parsing path variables). But theses aren't used/read from the result anywhere in django-environ.

environ/environ.py

@@ -785,21 +785,3 @@ def _absolute_join(base, *paths, **kwargs):
             raise ImproperlyConfigured(
                 "Create required path: {0}".format(absolute_path))
         return absolute_path
-
-
-def register_scheme(scheme):
-    for method in dir(urlparselib):
-        if method.startswith('uses_'):
-            getattr(urlparselib, method).append(scheme)
-
-
-def register_schemes(schemes):
-    for scheme in schemes:
-        register_scheme(scheme)
-
-
-# Register database and cache schemes in URLs.
-register_schemes(Env.DB_SCHEMES.keys())
-register_schemes(Env.CACHE_SCHEMES.keys())
-register_schemes(Env.SEARCH_SCHEMES.keys())
-register_schemes(Env.EMAIL_SCHEMES.keys())