Drop support of package_bundled_index.json and builtin_tools_versions.txt

[alessio-perugini]

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What kind of change does this PR introduce?

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: package_bundled_index.json and builtin_tools_versions.txt for good as the Java IDE is deprecated trying to maintain this legacy part doesn't bring any relevant value.

What is the current behavior?

What is the new behavior?

Does this PR introduce a breaking change, and is titled accordingly?

Other information

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (1911448) 68.46% compared to head (77c8204) 68.78%.

Files	Patch %	Lines
...al/arduino/cores/packagemanager/package_manager.go	44.44%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2424      +/-   ##
==========================================
+ Coverage   68.46%   68.78%   +0.31%     
==========================================
  Files         204      204              
  Lines       20553    20446     -107     
==========================================
- Hits        14071    14063       -8     
+ Misses       5332     5242      -90     
+ Partials     1150     1141       -9     

Flag	Coverage Δ
unit	68.78% <58.33%> (+0.31%)	⬆️

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[umbynos]

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[alessio-perugini]

@per1234 do you know if those are used in the real world? it was used in the java IDE but we are planning to remove the support for those.
https://arduino.github.io/arduino-cli/0.35/configuration/
directories:
- builtin.libraries - the libraries in this directory will be available to all platforms without the need for the user to install them, but with the lowest priority over other installed libraries with the same name, it's the equivalent of the Arduino IDE's bundled libraries directory.

• builtin.tools - it's a list of directories of tools that will be available to all platforms without the need for the user to install them, it's the equivalent of the Arduino IDE 1.x bundled tools directory.