Remove _load_one_plugin fallback

Remove the _load_one_plugin fallback for stevedore version < 0.14. The
commit c011d7360085dc5b2b2521e5c533ccc87b310f15 pinned the stevedore
version to >= 0.14.

Change-Id: I73ab108a55613d9b9f6ce577d7a90ffe6596bd2c
Merge "Remove _load_one_plugin fallback"