block: drop unused bdrv_clear_incoming_migration_all() prototype

The bdrv_clear_incoming_migration_all() function has not existed since
commit 7ea2d269cb84ca7a2f4b7c3735634176f7c1dc35 ("block/migration:
Disable cache invalidate for incoming migration").

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1418212937-22222-1-git-send-email-stefanha@redhat.com
