EVP: Make the KEYEXCH implementation leaner

EVP: Make the KEYEXCH implementation leaner

Because the algorithm to use is decided already when creating an
EVP_PKEY_CTX regardless of how it was created, it turns out that it's
unnecessary to provide the KEYEXCH method explicitly, and rather
always have it be fetched implicitly.

This means fewer changes for applications that want to use new key
exchange algorithms / implementations.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10305)