Remove remaining artifacts from dataflow apitools client

[jrmccluskey]

Remove the module level init file and the README that were in the old dataflow apitools client directory.

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[github-actions]

Assigning reviewers:

R: @damccorm for label python.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (master@aaad9b4). Learn more about missing BASE report.
⚠️ Report is 83 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #39439   +/-   ##
=========================================
  Coverage          ?   55.80%           
  Complexity        ?     2232           
=========================================
  Files             ?     1111           
  Lines             ?   174026           
  Branches          ?     1433           
=========================================
  Hits              ?    97109           
  Misses            ?    74475           
  Partials          ?     2442           

Flag	Coverage Δ
python	79.75% <ø> (?)

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[github-actions]

Reminder, please take a look at this pr: @damccorm