ParDo should only accept DoFns

[kennknowles]

for version 3.0

Remove support for accepting user fns directly as ParDo input.

Remove this function:

beam/sdks/python/apache_beam/transforms/core.py

Line 639 in 911bfbd

def make_fn(self, fn):

Before version 3.0 the above function could be deprecated.

Imported from Jira BEAM-2427. Original Jira may contain additional context.
Reported by: altay.