rx: Don't maintain maxSerial

rx: Don't maintain maxSerial

There were no users within the code of the rx connection maxSerial
element, and maintaining it required locking on a critical path. So,
get rid of it.

Reviewed-on: http://gerrit.openafs.org/4797
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit 6a22a2f4b7aa0d960cea17b419003986f6184d58)

Change-Id: I465e73b7cc7c57b384cd2652bbc8825a898b8bbc
Reviewed-on: http://gerrit.openafs.org/4930
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>