chore: remove transcribe_async.py (redundant) as last step in refactor.

[nicain]

This file was intentionally included as a part of the refactor in #223 in order to preserve a region tag used externally ("speech_transcribe_async_gcs"). That external dependency has been reorganized to point to transcribe_gcs_async.py, and so this file can be safely removed.

[snippet-bot]

You are about to delete the following sample browser pages.

• samples/snippets/transcribe_async.py:28, tag speech_transcribe_async(usage: page 1).
• samples/snippets/transcribe_async.py:71, tag speech_transcribe_async_gcs(usage: page 1).

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Here is the summary of changes.

You are about to delete 4 region tags.

• samples/snippets/transcribe_async.py:28, tag speech_transcribe_async
• samples/snippets/transcribe_async.py:35, tag speech_python_migration_async_request
• samples/snippets/transcribe_async.py:51, tag speech_python_migration_async_response
• samples/snippets/transcribe_async.py:71, tag speech_transcribe_async_gcs

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