docs: remove datalab migration doc

[Linchin]

Description

Merge after internal change 579341078 is submitted.
Unblocks #10891.

Because one of datalab dependencies (pandas-profiling) deleted the older versions that datalab is using, we delete the datalab migration sample here.

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.9 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to delete 16 region tags.

• bigquery/datalab-migration/samples_test.py:72, tag bigquery_migration_datalab_query_magic
• bigquery/datalab-migration/samples_test.py:96, tag bigquery_migration_client_library_query_magic
• bigquery/datalab-migration/samples_test.py:117, tag bigquery_migration_datalab_query_magic_define_query
• bigquery/datalab-migration/samples_test.py:127, tag bigquery_migration_datalab_execute_query
• bigquery/datalab-migration/samples_test.py:146, tag bigquery_migration_client_library_query_magic_results_variable
• bigquery/datalab-migration/samples_test.py:167, tag bigquery_migration_datalab_list_tables_magic
• bigquery/datalab-migration/samples_test.py:180, tag bigquery_migration_datalab_query
• bigquery/datalab-migration/samples_test.py:195, tag bigquery_migration_client_library_query
• bigquery/datalab-migration/samples_test.py:212, tag bigquery_migration_datalab_load_table_from_gcs_csv
• bigquery/datalab-migration/samples_test.py:221, tag bigquery_migration_datalab_load_table_from_gcs_csv
• bigquery/datalab-migration/samples_test.py:242, tag bigquery_migration_client_library_load_table_from_gcs_csv
• bigquery/datalab-migration/samples_test.py:253, tag bigquery_migration_client_library_load_table_from_gcs_csv
• bigquery/datalab-migration/samples_test.py:284, tag bigquery_migration_datalab_load_table_from_dataframe
• bigquery/datalab-migration/samples_test.py:294, tag bigquery_migration_datalab_load_table_from_dataframe
• bigquery/datalab-migration/samples_test.py:324, tag bigquery_migration_client_library_load_table_from_dataframe
• bigquery/datalab-migration/samples_test.py:335, tag bigquery_migration_client_library_load_table_from_dataframe

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[rsamborski]

Double checked couple random region_tags and it seems they are no longer published on c.g.c, so this should be fine to merge.