Use default branch of ArduinoCore-API in CI workflow

Previously, the `namespace_arduino` branch of `arduino/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
`arduino/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

@@ -87,8 +87,6 @@ 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