Use default branch of ArduinoCore-API in CI workflow (#41)

Previously, the "namespace_arduino" branch of ArduinoCore-API was used in the core library of the development version of
the Mbed OS Boards platform used in the "Compile Examples" CI workflow. The default branch of ArduinoCore-API is now the
latest and greatest version, so the "namespace_arduino" branch should no longer be used.
for the Mbed OS Boards platform's core library.

Author: per1234

.github/workflows/compile-examples.yml

@@ -53,8 +53,6 @@ jobs:
         uses: actions/checkout@v2
         with:
           repository: arduino/ArduinoCore-API
-          # As specified at https://github.com/arduino/ArduinoCore-mbed/blob/master/README.md#installation
-          ref: namespace_arduino
           path: ${{ env.ARDUINOCORE_API_STAGING_PATH }}
 
       - name: Install ArduinoCore-API