DEP: Drop unique_counts, use pandas.DataFrame.nunique instead

[Zeroto521]

pandas.DataFrame.nunique already did this.

• closes #xxxx
• whatsnew entry

[codecov]

Codecov Report

Merging #502 (a03c2c2) into main (9d71bb3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #502   +/-   ##
=======================================
  Coverage   99.64%   99.64%           
=======================================
  Files          60       60           
  Lines         834      836    +2     
  Branches       93       92    -1     
=======================================
+ Hits          831      833    +2     
  Misses          1        1           
  Partials        2        2           

Impacted Files	Coverage Δ
dtoolkit/accessor/dataframe/unique_counts.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9d71bb3...a03c2c2. Read the comment docs.