Do not respond to SYNC_COM_CHANNEL_CLOSE requests

Do not respond to SYNC_COM_CHANNEL_CLOSE requests

SYNC clients, when closing a channel, send a SYNC_COM_CHANNEL_CLOSE
command, and immediately disconnect before waiting for a response. So,
don't send a SYNC response, since by the time we send it, the other end
will most likely have already hung up, causing a lot of needless
'SYNC_putRes: write failed' messages whenever a SYNC channel is closed
(e.g. when a demand-salvage finishes).

Reviewed-on: http://gerrit.openafs.org/434
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>