stream_manager: remove map in favor of just process
map and process methods in StreamPipeline are reduntant.
We want users to call process, so that we can be sure to automate emitting of error events to a side output.
As discussed with @otto we'll just keep process for now. as it best expressed the desired semantic of
processing events and not returning None values.
This API semantic might change post release. In that case we might want to reintroduce map with
appropriate DeprecationWarnings.
Bug: T336488