Drop support of `package_bundled_index.json` and `builtin_tools_versions.txt`

[alessio-perugini]

Describe the request

In the codebase we have 2 comments that tracks the support of:

• package_bundled_index.json
• builtin_tools_versions.txt

After an internal discussion, we have decided that after the refactoring of the legacy builder, is impossible to maintain the old arduino-builder used by the Java IDE.
In the spirit of the upcoming v1.0.0 we're dropping the support of the above files for good as the Java IDE is deprecated trying to maintain this legacy part doesn't bring any relevant value.

Describe the current behavior

Arduino CLI version

Operating system

N/A

Operating system version

Additional context

No response

Issue checklist

• I searched for previous requests in the issue tracker
• I verified the feature was still missing when using the nightly build
• My request contains all necessary details