ARROW-2507: [Rust] Don't take a reference when not needed.
ARROW-2507: [Rust] Don't take a reference when not needed.

Since this is already a reference value, we don't need to borrow it again.

Author: Bruce Mitchener <bruce.mitchener@gmail.com>

Closes #1941 from waywardmonkeys/remove-useless-reference and squashes the following commits:

6f782ad1 <Bruce Mitchener> ARROW-2507:  Don't take a reference when not needed.

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