Remove misleading comment in TLS 1.3 key schedule code
Remove misleading comment in TLS 1.3 key schedule code

The implementation documentation of

```
   mbedtls_ssl_tls1_3_derive_early_secrets()
```

mentioned the PSK binder key, which is misleading because the
function doesn't actually calculate it.

Signed-off-by: Hanno Becker <4d488caff78d6184f59bbe6e137e34d4606b9fe8@arm.com>

Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls