Remove unnecessary installation of arduino:samd from CI workflow (#66)

Installing Arduino SAMD Boards is unnecessary and can mask issues caused by Adafruit SAMD Boards' platform.txt specifying different tools versions than what the package index's toolsDependencies field defines.

Author: per1234

.github/workflows/githubci.yml

@@ -36,7 +36,6 @@ jobs:
         arduino-cli config init
         arduino-cli core update-index
         arduino-cli core update-index --additional-urls $BSP_URL   
-        arduino-cli core install arduino:samd --additional-urls $BSP_URL
         arduino-cli core install adafruit:samd --additional-urls $BSP_URL
         arduino-cli core install arduino:avr --additional-urls $BSP_URL
         arduino-cli core install adafruit:avr --additional-urls $BSP_URL