Remove dependency on experiments=beam_fn_api flag from PortableRunner

The execution path of Python's PortableRunner depends on the experiments=beam_fn_api flag. This flag is set automatically but has been broken in the past, e.g. when the FlinkRunner was subclassed from the PortableRunner (BEAM-8037).

We should get rid of the flag, as the only supported execution path for the PortableRunner is the BeamFnApi. Perhaps we can move the flag to the Dataflow runners which may still depend on it.