Fix some libcudf calls to cudf::detail::gather

[davidwendt]

Description

Fixes a couple source files that were calling gather by type-dispatching directly to the internal column_gatherer functor instead of using the cudf::detail::gather function(s). This simplifies the code and improves maintenance. For example, extra code to resolve the null-mask is eliminated since the appropriate cudf::detail::gather call does this automatically.
No function has changed, just code cleanup.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[codecov]

Codecov Report

Base: 87.40% // Head: 88.14% // Increases project coverage by +0.73% 🎉

Coverage data is based on head (9e3e1c4) compared to base (f72c4ce).
Patch coverage: 90.03% of modified lines in pull request are covered.

❗ Current head 9e3e1c4 differs from pull request most recent head 7a82bf0. Consider uploading reports for the commit 7a82bf0 to get more accurate results

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-22.12   #11963      +/-   ##
================================================
+ Coverage         87.40%   88.14%   +0.73%     
================================================
  Files               133      133              
  Lines             21833    21982     +149     
================================================
+ Hits              19084    19375     +291     
+ Misses             2749     2607     -142     

Impacted Files	Coverage Δ
python/cudf/cudf/core/dataframe.py	93.77% <ø> (ø)
python/cudf/cudf/core/frame.py	93.68% <ø> (ø)
python/cudf/cudf/core/indexed_frame.py	92.03% <ø> (ø)
python/cudf/cudf/core/udf/__init__.py	97.05% <ø> (+47.05%)	⬆️
python/cudf/cudf/io/orc.py	92.94% <ø> (-0.09%)	⬇️
python/cudf/cudf/testing/dataset_generator.py	72.83% <ø> (-0.42%)	⬇️
...thon/dask_cudf/dask_cudf/tests/test_distributed.py	18.86% <ø> (+4.94%)	⬆️
python/cudf/cudf/core/_base_index.py	82.20% <43.75%> (-3.35%)	⬇️
python/cudf/cudf/io/text.py	91.66% <66.66%> (-8.34%)	⬇️
python/strings_udf/strings_udf/__init__.py	84.31% <76.00%> (-12.57%)	⬇️
... and 29 more

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[karthikeyann]

Great find! 👍

[davidwendt]

@gpucibot merge