remove not-an-error error message

[msimerson]

Changes proposed in this pull request:

• don't emit an error after deciding not to skip signing a message and logging why

Apr  2 07:48:29 [INFO] [] [dkim_verify] identity="@online.costco.com" domain="online.costco.com" selector="key1" result=pass
Apr  2 07:48:29 [NOTICE] [-] [dkim_sign] skipped: no private key for online.costco.com
Apr  2 07:48:29 [ERROR] [-] [core] missing key data for undefined.online.costco.com
Apr  2 07:48:30 [NOTICE] [] [core] disconnect ip=67.90.142.249 rdns=mta16v7.online.costco.com helo=mta16v7.online.costco.com relay=N early=N esmtp=Y tls=Y pipe=N errors=0 txns=1 rcpts=1/0/0 msgs=1/0/0 bytes=211031 lr="" time=8.286

Checklist:

• docs updated
• tests updated
• Changes updated