Remove unneeded Dataflow Runner v1 code.

[robertwb]

This is phase 1 of the cleanup.

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[robertwb]

R @tvalentyn

Do not merge until after 2.49 is cut.

[github-actions]

Checks are failing. Will not request review until checks are succeeding. If you'd like to override that behavior, comment assign set of reviewers

[codecov]

Codecov Report

Merging #27196 (3b0c329) into master (c70eb85) will decrease coverage by 0.41%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master   #27196      +/-   ##
==========================================
- Coverage   71.62%   71.22%   -0.41%     
==========================================
  Files         855      862       +7     
  Lines      104225   104627     +402     
==========================================
- Hits        74652    74518     -134     
- Misses      28026    28562     +536     
  Partials     1547     1547              

Flag	Coverage Δ
python	80.41% <92.85%> (-0.68%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdks/python/apache_beam/coders/coders.py	87.13% <ø> (+0.08%)	⬆️
...ks/python/apache_beam/coders/coders_test_common.py	98.55% <ø> (-0.03%)	⬇️
sdks/python/apache_beam/coders/row_coder.py	94.01% <ø> (-0.25%)	⬇️
...hon/apache_beam/runners/dataflow/internal/names.py	100.00% <ø> (ø)
...ache_beam/runners/dataflow/ptransform_overrides.py	100.00% <ø> (+1.33%)	⬆️
...on/apache_beam/runners/dataflow/dataflow_runner.py	83.33% <85.71%> (+1.86%)	⬆️
...apache_beam/runners/dataflow/internal/apiclient.py	75.63% <100.00%> (-1.39%)	⬇️

... and 44 files with indirect coverage changes

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[github-actions]

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

[tvalentyn]

waiting on author

[Abacn]

it appears there is some unused import breaking pythonlint precommit: https://ci-beam.apache.org/view/PostCommit/job/beam_PreCommit_PythonLint_Cron/5444/console

https://github.com/robertwb/incubator-beam/blob/3b0c3290b0b0978bbdb5c44aa025cc43076cbdb4/sdks/python/apache_beam/runners/dataflow/dataflow_runner.py#L46

[robertwb]

Huh. Jenkins was happy with it: https://ci-beam.apache.org/job/beam_PreCommit_PythonLint_Commit/20162/ Maybe a race. #27447