Remove references to legacy HMAC and AEAD demo programs

[davidhorstmann-arm]

Part of implementing #9904

TF-PSA-Crypto contains 2 particular PSA example programs: aead_demo.c and hmac_demo.c, that were written to demonstrate transitioning to the PSA API. These contain a description like the following:

/**
 * PSA API multi-part AEAD demonstration.
 *
 * This program AEAD-encrypts a message, using the algorithm and key size
 * specified on the command line, using the multi-part API.
 *
 * It comes with a companion program cipher/cipher_aead_demo.c, which does the
 * same operations with the legacy Cipher API. The goal is that comparing the
 * two programs will help people migrating to the PSA Crypto API.
 ...

The description refers to equivalent programs that use the legacy API, so as to demonstrate the transition.

In 4.0 we will remove the legacy programs, so we need to remove these references to them and rewrite the descriptions to be generic.

This task is done when the PSA example programs' descriptions do not make reference to the legacy programs.

Estimate: 1 Ideal Engineering Hour