Remove mentions of DSA from Mbed TLS

[gilles-peskine-arm]

The PSA headers and tests mention DSA (as in, finite-field DSA — not elliptic curves algorithms like ECDSA and EdDSA). In 2023 I can confidently state that we are not going to support it in Mbed TLS. Furthermore, supporting DSA in the PSA crypto API would require extensions to the API and not just an algorithm encoding, because DSA has unusual characteristics: it has partially independent sizes for the private key and the public key so key generation needs two bit-sizes and not just one; and it requires domain parameters for which we no longer plan to add an API (and in Mbed TLS we are going to remove the partial implementation of domain parameters).

The goal of this task is to remove the definitions of key types and algorithms for DSA as well as related macros and tests. However note these limitations:

• Technically, it's an API break to remove the publicly documented macros, so we may need to keep them until 4.0. We can freeze the macros and remove associated tests though (as long as we don't run into some test completeness issue). We can also remove the handling of DSA in crypto_sizes.h.
• The DSA definitions are just as useless in 2.28 as in 3.x. So we may want to do the same thing in 2.28 and 3.x. If we don't modify 2.28, we may want to keep the code to skip DSA in generate_psa_tests.py.