chore: Remove unused Ibis Python to JS UDF compiler

[tswast]

The Python to JavaScript compiler previously vendored from ibis and located at third_party/bigframes_vendored/ibis/backends/bigquery/udf was unused in the production codebase and not supported by the vendored backend. This PR removes the udf/ directory and any references to PythonToJavaScriptTranslator and _get_udf_source from third_party/bigframes_vendored/ibis/backends/bigquery/__init__.py.

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PR created automatically by Jules for task 17001767655361890847 started by @tswast

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