TLS ECDH 1: clean up existing client-side support

[mpg]

We currently support client-side ECDHE-ECDSA and ECDHE-RSA using PSA. However, the code implementing it could be simplified now that PSA is more stable. This task is about doing that, before other tasks build on the existing support and use it as an example.

• parameter check:
  • remove meaningless remnants of the era where conf curves was optional
  • simplify with the new conf group interface
  • do it for PSA too! (check TLS 1.3 too)
• trivial transcoding functions can be removed now that PSA formats are stable, as well as some intermediate buffers
• perhaps more PSA macros can be used for computing buffer sizes