Remove (long) obsolete declaration of unsupported features that are now supported. (#33239)

Author: robertwb

website/www/site/content/en/documentation/sdks/python-streaming.md

@@ -155,11 +155,3 @@ about executing streaming pipelines:
 - [DirectRunner streaming execution](/documentation/runners/direct/#streaming-execution)
 - [DataflowRunner streaming execution](/documentation/runners/dataflow/#streaming-execution)
 - [Portable Flink runner](/documentation/runners/flink/)
-
-## Unsupported features
-
-Python streaming execution does not currently support the following features:
-
-- Custom source API
-- User-defined custom merging `WindowFn` (with fnapi)
-- For portable runners, see [portability support table](https://s.apache.org/apache-beam-portability-support-table).