[BEAM-11822] Revert "[BEAM-2914] Add portable merging window support to Python. (#12995)"

[ajamato]

This reverts commit 625ee1f.

The commit broke ValidateRunnerXlangTest.combineGloballyTest broken in beam_PostCommit_XVR_Direct

See test failure details here:
https://issues.apache.org/jira/browse/BEAM-11822?filter=-2

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Post-Commit Tests Status (on master branch)

Lang	SDK	Dataflow	Flink	Samza	Spark	Twister2
Go		---		---		---
Java
Python				---		---
XLang				---		---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website	Whitespace	Typescript
Non-portable
Portable	---		---	---	---	---

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GitHub Actions Tests Status (on master branch)

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[ajamato]

R: @robertwb

[robertwb]

Pure rollback? If so, LGTM.

[codecov]

Codecov Report

Merging #14004 (9c60fd5) into master (8f87957) will decrease coverage by 0.02%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master   #14004      +/-   ##
==========================================
- Coverage   82.94%   82.92%   -0.03%     
==========================================
  Files         469      469              
  Lines       58343    58233     -110     
==========================================
- Hits        48394    48287     -107     
+ Misses       9949     9946       -3     

Impacted Files	Coverage Δ
...hon/apache_beam/runners/worker/bundle_processor.py	93.95% <ø> (+0.09%)	⬆️
...eam/runners/portability/fn_api_runner/execution.py	92.03% <42.85%> (-1.65%)	⬇️
...dks/python/apache_beam/runners/pipeline_context.py	92.74% <0.00%> (-0.81%)	⬇️
sdks/python/apache_beam/coders/coder_impl.py	95.38% <0.00%> (-0.14%)	⬇️
...ks/python/apache_beam/runners/worker/sdk_worker.py	89.69% <0.00%> (+0.15%)	⬆️
sdks/python/apache_beam/io/iobase.py	84.81% <0.00%> (+0.26%)	⬆️
sdks/python/apache_beam/io/localfilesystem.py	92.42% <0.00%> (+0.75%)	⬆️
sdks/python/apache_beam/internal/metrics/metric.py	87.50% <0.00%> (+1.04%)	⬆️

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