ARROW-6475: [C++] Don't try to dictionary encode dictionary arrays
ARROW-6475: [C++] Don't try to dictionary encode dictionary arrays

With #5077 (or possibly #4949) behavior with dictionary arrays changed, leaving the explicit call to DictionaryEncode() redundant @wesm

Closes #5299 from bkietz/6434-Crossbow-Nightly-HDFS-int and squashes the following commits:

b29e6b732 <Benjamin Kietzman> don't try to dictionary encode dictionary arrays

Authored-by: Benjamin Kietzman <be26f8d5d161e4d7965ee00602589fb2b74149f2@gmail.com>
Signed-off-by: Krisztián Szűcs <6bab32dc957a05a87930e779f272342e159326fa@gmail.com>

xhochy/arrow,icexelloss/arrow,cpcloud/arrow,renesugar/arrow,wesm/arrow,wesm/arrow,xhochy/arrow,apache/arrow,icexelloss/arrow,apache/arrow,laurentgo/arrow,laurentgo/arrow,xhochy/arrow,apache/arrow,apache/arrow,xhochy/arrow,laurentgo/arrow,cpcloud/arrow,xhochy/arrow,icexelloss/arrow,laurentgo/arrow,kou/arrow,renesugar/arrow,xhochy/arrow,cpcloud/arrow,xhochy/arrow,laurentgo/arrow,kou/arrow,xhochy/arrow,kou/arrow,renesugar/arrow,xhochy/arrow,renesugar/arrow,apache/arrow,icexelloss/arrow,apache/arrow,kou/arrow,renesugar/arrow,icexelloss/arrow,renesugar/arrow,laurentgo/arrow,xhochy/arrow,renesugar/arrow,wesm/arrow,cpcloud/arrow,apache/arrow,wesm/arrow,cpcloud/arrow,xhochy/arrow,apache/arrow,icexelloss/arrow,cpcloud/arrow,cpcloud/arrow,wesm/arrow,laurentgo/arrow,kou/arrow,renesugar/arrow,icexelloss/arrow,laurentgo/arrow,wesm/arrow,kou/arrow,wesm/arrow,wesm/arrow,kou/arrow,renesugar/arrow,xhochy/arrow,icexelloss/arrow,wesm/arrow,cpcloud/arrow,laurentgo/arrow,kou/arrow,laurentgo/arrow,cpcloud/arrow,kou/arrow,kou/arrow,renesugar/arrow,icexelloss/arrow,icexelloss/arrow,laurentgo/arrow,apache/arrow,kou/arrow,cpcloud/arrow,apache/arrow,renesugar/arrow,renesugar/arrow,icexelloss/arrow,icexelloss/arrow,icexelloss/arrow,renesugar/arrow,wesm/arrow,cpcloud/arrow,cpcloud/arrow,xhochy/arrow,kou/arrow,cpcloud/arrow,wesm/arrow,cpcloud/arrow,wesm/arrow,apache/arrow,renesugar/arrow,xhochy/arrow,laurentgo/arrow,apache/arrow,apache/arrow,xhochy/arrow,apache/arrow,laurentgo/arrow,laurentgo/arrow,wesm/arrow,kou/arrow,kou/arrow,wesm/arrow,renesugar/arrow,icexelloss/arrow,laurentgo/arrow