Remove code for unnecessary Adafruit SAMD Boards platform dependency

[per1234]

Previously, Arduino SAMD Boards was installed whenever the script was run for one of the Adafruit SAMD Boards.

Adafruit SAMD Boards doesn't have a dependency on Arduino SAMD Boards, so this only slows down the CI run.

• Describe the scope of your change--i.e. what the change does and what parts
  of the code were modified. This will help us understand any risks of integrating
  the code.

I removed the code that installs Arduino SAMD Boards whenever the code is being compiled for one of the Adafruit SAMD Boards.

• Describe any known limitations with your change. For example if the change
  doesn't apply to a supported platform of the library please mention it.

This change would only cause a problem if Adafruit SAMD Boards changed to having a dependency on Arduino SAMD Boards at some point in the future.

• Please run any tests or examples that can exercise your modified code. We
  strive to not break users of the code and running tests/examples helps with this
  process.

I have tested this in my fork:
https://github.com/per1234/ci-arduino/runs/745991258?check_suite_focus=true#step:5:28
I notice this repository's GitHub Actions workflow doesn't test the script for any Adafruit boards. If you would like me to add one of the Adafruit SAMD Boards to the workflow to verify this PR, I'm happy to do so.

[ladyada]

thanks will try it! :)