ARROW-1144: [C++] Remove unused variable
ARROW-1144: [C++] Remove unused variable

Author: Phillip Cloud <cpcloud@gmail.com>

Closes #774 from cpcloud/ARROW-1144 and squashes the following commits:

a79aebaa [Phillip Cloud] ARROW-1144: [C++] Remove unused variable

apache/arrow,majetideepak/arrow,xhochy/arrow,tebeka/arrow,yufeldman/arrow,wesm/arrow,apache/arrow,apache/arrow,icexelloss/arrow,pcmoritz/arrow,itaiin/arrow,renesugar/arrow,icexelloss/arrow,laurentgo/arrow,apache/arrow,siddharthteotia/arrow,itaiin/arrow,pcmoritz/arrow,NonVolatileComputing/arrow,xhochy/arrow,kou/arrow,yufeldman/arrow,laurentgo/arrow,NonVolatileComputing/arrow,laurentgo/arrow,laurentgo/arrow,renesugar/arrow,wagavulin/arrow,dremio/arrow,pcmoritz/arrow,icexelloss/arrow,NonVolatileComputing/arrow,kou/arrow,siddharthteotia/arrow,itaiin/arrow,itaiin/arrow,yufeldman/arrow,icexelloss/arrow,majetideepak/arrow,dremio/arrow,cpcloud/arrow,tebeka/arrow,renesugar/arrow,pcmoritz/arrow,tebeka/arrow,dremio/arrow,majetideepak/arrow,cpcloud/arrow,apache/arrow,pcmoritz/arrow,wesm/arrow,kou/arrow,dremio/arrow,laurentgo/arrow,cpcloud/arrow,laurentgo/arrow,apache/arrow,apache/arrow,dremio/arrow,jeffknupp/arrow,xhochy/arrow,wagavulin/arrow,NonVolatileComputing/arrow,siddharthteotia/arrow,yufeldman/arrow,pcmoritz/arrow,jeffknupp/arrow,wesm/arrow,renesugar/arrow,cpcloud/arrow,renesugar/arrow,cpcloud/arrow,wesm/arrow,wagavulin/arrow,pcmoritz/arrow,yufeldman/arrow,itaiin/arrow,tebeka/arrow,yufeldman/arrow,laurentgo/arrow,majetideepak/arrow,tebeka/arrow,renesugar/arrow,xhochy/arrow,wagavulin/arrow,xhochy/arrow,xhochy/arrow,kou/arrow,wesm/arrow,siddharthteotia/arrow,apache/arrow,pcmoritz/arrow,dremio/arrow,renesugar/arrow,laurentgo/arrow,kou/arrow,pcmoritz/arrow,itaiin/arrow,itaiin/arrow,xhochy/arrow,jeffknupp/arrow,pcmoritz/arrow,cpcloud/arrow,majetideepak/arrow,xhochy/arrow,majetideepak/arrow,dremio/arrow,apache/arrow,wesm/arrow,jeffknupp/arrow,jeffknupp/arrow,NonVolatileComputing/arrow,cpcloud/arrow,icexelloss/arrow,dremio/arrow,pcmoritz/arrow,wesm/arrow,majetideepak/arrow,xhochy/arrow,apache/arrow,wagavulin/arrow,wagavulin/arrow,laurentgo/arrow,siddharthteotia/arrow,renesugar/arrow,siddharthteotia/arrow,jeffknupp/arrow,wagavulin/arrow,wagavulin/arrow,jeffknupp/arrow,xhochy/arrow,itaiin/arrow,renesugar/arrow,itaiin/arrow,pcmoritz/arrow,cpcloud/arrow,itaiin/arrow,wagavulin/arrow,siddharthteotia/arrow,siddharthteotia/arrow,majetideepak/arrow,wagavulin/arrow,wesm/arrow,icexelloss/arrow,kou/arrow,NonVolatileComputing/arrow,renesugar/arrow,majetideepak/arrow,cpcloud/arrow,laurentgo/arrow,itaiin/arrow,kou/arrow,yufeldman/arrow,dremio/arrow,itaiin/arrow,itaiin/arrow,xhochy/arrow,tebeka/arrow,NonVolatileComputing/arrow,xhochy/arrow,apache/arrow,renesugar/arrow,NonVolatileComputing/arrow,tebeka/arrow,majetideepak/arrow,pcmoritz/arrow,icexelloss/arrow,cpcloud/arrow,kou/arrow,majetideepak/arrow,laurentgo/arrow,majetideepak/arrow,siddharthteotia/arrow,apache/arrow,NonVolatileComputing/arrow,NonVolatileComputing/arrow,icexelloss/arrow,jeffknupp/arrow,xhochy/arrow,laurentgo/arrow,dremio/arrow,pcmoritz/arrow,renesugar/arrow,icexelloss/arrow,kou/arrow,yufeldman/arrow,yufeldman/arrow,wagavulin/arrow,cpcloud/arrow,laurentgo/arrow,laurentgo/arrow,itaiin/arrow,kou/arrow,wesm/arrow,tebeka/arrow,renesugar/arrow,renesugar/arrow,NonVolatileComputing/arrow,kou/arrow,majetideepak/arrow,siddharthteotia/arrow,yufeldman/arrow,wesm/arrow,wesm/arrow,wesm/arrow,xhochy/arrow,icexelloss/arrow,icexelloss/arrow,icexelloss/arrow,cpcloud/arrow,jeffknupp/arrow,itaiin/arrow,dremio/arrow,tebeka/arrow,siddharthteotia/arrow,tebeka/arrow,jeffknupp/arrow,kou/arrow,jeffknupp/arrow,wesm/arrow,laurentgo/arrow,kou/arrow,apache/arrow,icexelloss/arrow,pcmoritz/arrow,renesugar/arrow,cpcloud/arrow,majetideepak/arrow,apache/arrow,icexelloss/arrow,cpcloud/arrow,xhochy/arrow,kou/arrow,yufeldman/arrow,majetideepak/arrow,wesm/arrow,wagavulin/arrow