Revert "dev/settings_base: don't remove failed mc hosts."

This reverts commit 1b6d6e48ef8e84a7ab3ad6d2373e285a25fb03ee.

Revert "dev/settings_base: python-memcached -> pylibmc."

This reverts commit e87174111733568847719b6a73f6ad700fda8a1e.