[C++][Dataset] Simplify parquet column projection

[asfimport]

This is a minor makeup.

Reporter: Francois Saint-Jacques / @fsaintjacques
Assignee: Francois Saint-Jacques / @fsaintjacques

PRs and other links:

• GitHub Pull Request #6018

_{Note: This issue was originally created as ARROW-7377. Please see the migration documentation for further details.}

[asfimport]

Ben Kietzman / @bkietz:
Issue resolved by pull request 6018
#6018