Do not complete pending gmirror BIOs when tearing down the provider.

This will result in lock recursion and is more generally incorrect since
the completion handlers will just reinsert the BIOs into the queue we're
trying to drain.

Reviewed by: imp, ngie
Approved by: re (gjb)
MFC after: 3 weeks
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D6908