[C++/Python] Reduce some duplicated code in python/builtin_convert.cc

[asfimport]

See discussion in #2270

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

_{Note: This issue was originally created as ARROW-2865. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
This was achieved in ARROW-2814