Remove obsolete cudf::strings::replace_nulls

[davidwendt]

The cudf::strings::replace_nulls() is a public API that was replaced by cudf::replace_nulls().
The strings one should not be used since the base one handles any column type.

[codecov]

Codecov Report

Merging #7965 (44ed039) into branch-0.20 (599f62d) will increase coverage by 0.02%.
The diff coverage is 92.08%.

❗ Current head 44ed039 differs from pull request most recent head cf2fb70. Consider uploading reports for the commit cf2fb70 to get more accurate results

@@               Coverage Diff               @@
##           branch-0.20    #7965      +/-   ##
===============================================
+ Coverage        82.30%   82.32%   +0.02%     
===============================================
  Files              101      103       +2     
  Lines            17053    17374     +321     
===============================================
+ Hits             14035    14303     +268     
- Misses            3018     3071      +53     

Impacted Files	Coverage Δ
python/cudf/cudf/utils/dtypes.py	83.44% <ø> (-6.45%)	⬇️
python/cudf/cudf/utils/utils.py	82.85% <ø> (-2.21%)	⬇️
python/cudf/cudf/core/column/lists.py	86.95% <80.00%> (-0.27%)	⬇️
python/cudf/cudf/core/groupby/groupby.py	91.27% <80.95%> (-2.18%)	⬇️
python/cudf/cudf/core/column/struct.py	96.15% <86.66%> (-3.85%)	⬇️
python/cudf/cudf/core/index.py	92.58% <88.09%> (-0.44%)	⬇️
python/cudf/cudf/core/column/column.py	87.77% <88.57%> (+0.33%)	⬆️
python/cudf/cudf/core/column/decimal.py	92.92% <91.48%> (-0.92%)	⬇️
python/cudf/cudf/core/column/interval.py	90.69% <92.30%> (+0.07%)	⬆️
python/cudf/cudf/core/join/join.py	96.72% <93.75%> (+0.20%)	⬆️
... and 25 more

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[codereport]

lgtm

[davidwendt]

@gpucibot merge