GOVERNANCE: Drop the co-sponsor requirement

In OCI votes to date [1–11], I have only seen a co-sponsor in [9].  I
haven't been able to turn up local motivation for this requirement,
and don't expect rogue maintainers to flood the list with spam motions
now that the co-sponsor requirement has been lifted ;).

[1]: https://groups.google.com/a/opencontainers.org/d/msg/tob/WXk1uTgfXrs/-xICOdi1BwAJ
     Subject: Proposal for a new project: OCI Image Format Spec
     Date: Fri, 26 Feb 2016 00:33:07 +0000
     Message-ID: <CAD2oYtPCZRUXOER-Fz7WKsnNwYA-PoPmCG59jD1i3=xgYnGLZw@mail.gmail.com>
[2]: https://groups.google.com/a/opencontainers.org/d/msg/tob/h2x5aLVX2PY/qpry5QXABAAJ
     Subject: Proposal for a new project: OCI Image Format Spec (v2)
     Date: Fri, 18 Mar 2016 05:01:17 +0000
     Message-ID: <CAD2oYtMRhJhGBDwt77_L7Ac4LJ22Zsys+Q=zJLvYA1dwcmLE0w@mail.gmail.com>
[3]: https://groups.google.com/a/opencontainers.org/d/msg/tob/KGyPpu8YfNk/JnOvOEdSBQAJ
     Subject: ACTION: Proposal for a new project: OCI Image Format Spec (v3)
     Date: Sun, 20 Mar 2016 01:41:27 +0000
     Message-ID: <CAD2oYtNiOgCgLgZk--4bfrdywTmwP=bqSxvkSJ-rv=PTEoaTag@mail.gmail.com>
[4]: https://groups.google.com/a/opencontainers.org/d/msg/dev/XXxKY7w4YA4/KLgQX6iSAQAJ
     Subject: Proposal: Rename Image Spec milestone v1.0.0-rc1 to v0.3.0
     Date: Wed, 15 Jun 2016 23:35:12 +0000
     Message-ID: <CAD2oYtPoAvY8JTrz2Yi1zwjOyOuRR5faY=9jH=_QmsS_rwwbFA@mail.gmail.com>
[5]: https://groups.google.com/a/opencontainers.org/d/msg/dev/ik3MIDWq4Us/AqS5GUfuAwAJ
     Subject: Vote Required: OCI Image Spec Release Process
     Date: Thu, 23 Jun 2016 15:56:40 +0000
     Message-ID: <CAD2oYtNnW+hP7Q3NPBdYHOKfigU0pvbgcphKPhRB=ZfQBwX8VA@mail.gmail.com>
[6]: https://groups.google.com/a/opencontainers.org/d/msg/dev/a90Nx4Md6R4/KG2WiSr3AwAJ
     Subject: Vote Required: OCI runC Release Process
     Date: Thu, 23 Jun 2016 18:39:32 +0000
     Message-ID: <CAD2oYtOOB7zn-Bhp2Xgt+Wv4u=+_RAgEt=ic+60Qf=Z3j0WbQQ@mail.gmail.com>
[7]: https://groups.google.com/a/opencontainers.org/d/msg/dev/FoYcImNQg4c/Re0guPPHBQAJ
     Subject: [image-spec VOTE]: Tag ca58cdf as v0.3.0 (closes 2016-07-06 17:00 UTC)
     Date: Wed, 29 Jun 2016 16:36:49 +0000
     Message-ID: <CAD2oYtNUMZ_E=NyvCq9_N=SLW75aR1QkLsb0+Uz6aSQr8c0-0Q@mail.gmail.com>
[8]: https://groups.google.com/a/opencontainers.org/d/msg/tob/ftfzwhD6DpA/QFCi3siaDQAJ
     Subject: [project-template VOTE]: Merge governance and release docs at
       56abe12 (closes 2016-07-19 19:00 UTC)
     Date: Tue, 12 Jul 2016 19:26:40 +0000
     Message-ID: <CAD2oYtOVOGJFPG43eXty_dWudywXpWVQAa1vXQT004TrQ8ze5w@mail.gmail.com>
[9]: https://groups.google.com/a/opencontainers.org/d/msg/dev/XwIb5czltSE/GKs-6FkGCgAJ
     Subject: [image-spec VOTE]: Tag 2bae0dc as v0.4.0 (closes 2016-08-07 16:20 UTC)
     Date: Wed, 10 Aug 2016 12:22:33 -0400
     Message-ID: <CAN6Zp5wJX9+7GFdf0=8A9Ms8grvo9sk0wE16wBbci5qUPXXFag@mail.gmail.com>
[10]: https://groups.google.com/a/opencontainers.org/d/msg/dev/fiDgBKTR9f4/a6B3C-F6DAAJ
      Subject: [runc VOTE]: Tag 6df383c2f881fbdeb as 1.0.0-rc2
      Date: Thu, 18 Aug 2016 09:20:25 -0700
      Message-ID: <CADe0YkvxkFwF_HepCmz2iWENh3v2TVuYSECStdjeVgnGXKLM8Q@mail.gmail.com>
[11]: https://groups.google.com/a/opencontainers.org/d/msg/dev/5qj2hATVxew/T9xFOcO-EAAJ
      Subject: [runtime-spec VOTE]: Tag d3c3763b as v1.0.0-rc2 (closes 2016-09-08
        13:47 UTC)
      Date: Thu, 1 Sep 2016 09:49:24 -0400
      Message-ID: <CANcsyS7exPPW17=o0qn6XHKesWm1h_599Fic5rBefkQgTdf4kA@mail.gmail.com>

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

GOVERNANCE.md

@@ -8,7 +8,7 @@ This section describes generic rules and procedures for fulfilling that mandate.
 
 ## Proposing a motion
 
-A maintainer SHOULD propose a motion on the [email protected] mailing list (except [security issues](#security-issues)) with another maintainer as a co-sponsor.
+A maintainer SHOULD propose a motion on the [email protected] mailing list (except [security issues](#security-issues)).
 
 ## Voting