[C++] Don't try to dictionary encode dictionary arrays

[asfimport]

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

Reporter: Krisztian Szucs / @kszucs
Assignee: Ben Kietzman / @bkietz

PRs and other links:

• GitHub Pull Request #5299
• GitHub Pull Request #5302

_{Note: This issue was originally created as ARROW-6475. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 5299
#5299