Removed BaseCommand.stdin introduced in 116d39842d.

This option is not actually very useful in the general case
because it doesn't override sys.stdin.

It's still marginally useful for testing some features of
the createsuperuser command so it was moved there.
This commit also makes the detection of a TTY in createsuperuser
a bit more robust, after a suggestion of appolo13.

django/core/management/base.py

@@ -306,7 +306,6 @@ class BaseCommand(object):
             self.stderr = OutputWrapper(options.get('stderr', sys.stderr))
         else:
             self.stderr = OutputWrapper(options.get('stderr', sys.stderr), self.style.ERROR)
-        self.stdin = options.get('stdin', sys.stdin)  # Useful for testing
 
         if self.can_import_settings:
             from django.conf import settings  # NOQA