CMake: Remove duplicate documents
CMake: Remove duplicate documents

- Remove the readme as it is a duplicate content of official document
  https://os.mbed.com/docs/mbed-os/v6.9/debug-test/unit-testing.html

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed