Revert "resend pr for bug 948826"

This reverts commit 0214caef6fd00eb5ae5f439d911766ba4c15ed2d.
Bug 958105 - [Messages][Drafts] Restore thread timestamp with last message timestamp when draft is discarded

https://bugzilla.mozilla.org/show_bug.cgi?id=958105
Signed-off-by: Rick Waldron <waldron.rick@gmail.com>