Remove the SSL API mbedtls_ssl_get_session_pointer()

This affects two classes of users:

  1. Users who manually inspect parts of the current session through direct structure field access.

  2. Users of session resumption who query the current session via mbedtls_ssl_get_session_pointer() prior to saving or exporting it via mbedtls_ssl_session_copy() or mbedtls_ssl_session_save(), respectively.

Migration paths:

  1. Mbed TLS 3.0 does not offer a migration path for the usecase 1: Like many other Mbed TLS structures, the structure of mbedtls_ssl_session is no longer part of the public API in Mbed TLS 3.0, and direct structure field access is no longer supported. Please see the corresponding migration guide.

  2. Users should replace calls to mbedtls_ssl_get_session_pointer() by calls to mbedtls_ssl_get_session() as demonstrated in the example program programs/ssl/ssl_client2.c.