ARROW-2272: [Python] Clean up leftovers in test_plasma.py
ARROW-2272: [Python] Clean up leftovers in test_plasma.py

Author: Antoine Pitrou <antoine@python.org>

Closes #1715 from pitrou/ARROW-2272-test-plasma-tmp and squashes the following commits:

6d6d084 <Antoine Pitrou> ARROW-2272:  Clean up leftovers in test_plasma.py

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