Drop very old schema_version table if it exists

Summary: Ref T1191. This predates the mdoern patch stuff and may exist on very, very old installs. By the time they apply this patch, it's guaranteed it won't matter anymore. Drop it to make the schemata consistent with expectations.

Test Plan: Ran patch on installs with and without the table.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D10611