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