Remove the option to build the library without any entropy sources

This does not affect users who use the default mbedtls_config.h, as this option was already off by default.

If you were using the MBEDTLS_TEST_NULL_ENTROPY option and your platform doesn't have any entropy source, you should use MBEDTLS_ENTROPY_NV_SEED and make sure your device is provisioned with a strong random seed. Alternatively, for testing purposes only, you can create and register a fake entropy function.