Remove previous whatsnewentry (from v0.25.1)

Author: Marco Gorelli

doc/source/whatsnew/v0.25.1.rst

@@ -90,7 +90,6 @@ Groupby/resample/rolling
 - Bug in :meth:`pandas.core.groupby.GroupBy.nth` where ``observed=False`` was being ignored for Categorical groupers (:issue:`26385`)
 - Bug in windowing over read-only arrays (:issue:`27766`)
 - Fixed segfault in `pandas.core.groupby.DataFrameGroupBy.quantile` when an invalid quantile was passed (:issue:`27470`)
-- Bug in :meth:`pandas.core.groupby.SeriesGroupBy.nunique` where ``NaT`` values were interfering with the count of unique values (:issue:`27951`)
 
 Reshaping
 ^^^^^^^^^