Remove use of DRBG modules from sample programs

[mpg]

Most of our sample programs still use the old pattern of directly using entropy and drbg modules. These days, they're supposed to just call psa_crypto_init() and then use the PSA RNG:

• either directly with psa_generate_random();
• or indirectly via mbedtls_psa_get_random() for functions that need an RNG callback.

Programs under test (selftests, benchmark) are an exception: they're allowed to still use drbg for the purpose of directly testing/benchmarking it.

Target programs (might want to split into 1 PR per directory if doing all in one go is a bit much for a single PR):

• ssl
• x509
• fuzz (one occurrence in the crypto repo too)

If entropy and drbg were the only internal thing used, also remove #define MBEDTLS_DECLARE_PRIVATE_IDENTIFIERS from the program while at it. If they were not the only thing, ideally create a follow-up issue about what remains.