Revert "radosgw-admin: Test bucket list for bucket starting with underscore."

This reverts commit fa70eb8f67371568f47ae237606be63024164214.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 1b0f241dcb7f4a03c3c5095464e740e5dd75831a)