[C++] Clean up unnecessary decimal module refs

[asfimport]

See this comment: #1610 (comment)

Reporter: Phillip Cloud / @cpcloud

_{Note: This issue was originally created as ARROW-2164. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
I closed this as there isn't any activity around it and it does not seem like a worthwhile us of our time.