Remove legacy API usage from ssl_server2.c

[davidhorstmann-arm]

Part of implementing #9904

Remove uses of the legacy crypto API from ssl_server2.c. Changes needed are:

• Remove the option to set custom DHM parameters with the dhm_file option. This removal is needed because the mbedtls_dhm functions are going away.
• Rewrite the ssl_async_resume() function to remove the ASYNC_OP_DECRYPT operation and thereby remove the use of mbedtls_pk_sign(). This is going away anyway in Remove the RSA-decryption key exchange #9682 so there is no problem with removing it.

This task is done when ssl_server2.c no longer calls any legacy function.

Estimate: 6 Ideal Engineering Hours

[gilles-peskine-arm]

Regarding ssl_async_resume: we're removing RSA-decryption authentication. As part of that, the mbedtls_ssl_async_decrypt_t callback is going away. So ssl_async_resume will no longer need to handle decryption.