Remove DataFrame.quantiles from docs. (#12684)

Removes documentation for `DataFrame.quantiles`, which was deprecated and then removed in #12281. This issue was discovered while building the docs in #12592.

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - AJ Schmidt (https://github.com/ajschmidt8)

URL: #12684

docs/cudf/source/api_docs/dataframe.rst

@@ -146,7 +146,6 @@ Computations / descriptive stats
    DataFrame.prod
    DataFrame.product
    DataFrame.quantile
-   DataFrame.quantiles
    DataFrame.rank
    DataFrame.round
    DataFrame.skew