chore: remove defunct _py_toolchain_type py_binary attribute

[rickeylev]

The _py_toolchain_type attribute is a holdover from when the rules were implemented
in Java and some Google-internal tests were relying on it.

[gemini-code-assist]

Summary of Changes

Hello @rickeylev, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses technical debt by removing an obsolete and unused attribute related to Python toolchains. The _py_toolchain_type attribute, identified as a vestige from a previous implementation, has been completely removed, simplifying the codebase and cleaning up unnecessary dependencies.

Highlights

• Code Cleanup: Removed the defunct _py_toolchain_type attribute from py_binary as it was a holdover from an older implementation and no longer necessary.
• Dependency Optimization: Eliminated the import of TARGET_TOOLCHAIN_TYPE in py_executable.bzl since it is no longer referenced after the removal of the _py_toolchain_type attribute.

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[gemini-code-assist]

Code Review

This pull request removes the defunct _py_toolchain_type attribute from py_binary rules, which is a good cleanup of legacy code. The changes are straightforward, removing the attribute definition and the corresponding unused import of TARGET_TOOLCHAIN_TYPE. This improves code clarity and maintainability by removing obsolete code. The changes look correct and I have no further suggestions.