Remove internal_arithmetic (not used)

Change-Id: I6c42495a7b912bae37c0f1fd0e161f7db240b755
Reviewed-on: http://review.couchbase.org/42599
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>