Remove the option to build the library without any entropy sources

This does not affect users who use the default 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.