Change bin/storage destroy to be less scary when removing test data
Summary: I've pulled up this code probably three different times to make sure that the big scary warning does, in fact, still get printed even when passing `--unitest-fixtures` to `bin/storage destroy`. Make the warning message less scary if only removing test data.
Test Plan: Ran with and without `--unitest-fixtures` and saw expected warnings. After agreeing to warnings, test data was deleted as expected. Did not test `bin/storage destroy` without `--unittest-fixtures`.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D19535