Remove "metamta.precedence-bulk" option (always enable it)

Summary: Ref T7746. This might possibly improve deliverability. Or might make it worse. Who knows?

Test Plan: grep

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7746

Differential Revision: https://secure.phabricator.com/D12266