Remove unused variable

couch-kvstore.cc:776:14: warning: variable 'newHeaderPos' set but not used
                         [-Wunused-but-set-variable]
     uint64_t              newHeaderPos = 0;

Change-Id: I8a7a6dd4997be011cadbe7db990b1a38aeffa4b3
Reviewed-on: http://review.couchbase.org/42231
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>