packet-couchbase.c: remove field which does not exists

Change-Id: Ia4a2a6127edbb6a92becf22cd6ee689e5e2a7966
Reviewed-on: https://code.wireshark.org/review/5981
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>