Fragile mbedtls configuration

[gilles-peskine-arm]

third_party/mbedtls/mbedtls-config.h has the following code:

#include "mbedtls/version.h"
// Include in the legacy config name adjustment file for mbedtls >= 3.5.0
#if (MBEDTLS_VERSION_NUMBER >= 0x03050000)
    #include "mbedtls/config_adjust_legacy_crypto.h"
#endif

This is an unintended workaround and might not compile with Mbed TLS 3.6.1. I think that the OpenThread configuration is simple enough that there's currently no problem with inconsistencies, but it's at least fragile.

The solution is to remove this block and also remove #include "mbedtls/check_config.h", which since Mbed TLS 3.0 is done automatically at the right point.

[jwhui]

@gilles-peskine-arm , thanks for raising this issue.

I just submitted #10253. Please review if you get a chance.