EVP: Make the SIGNATURE 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 SIGNATURE method explicitly, and rather
always have it be fetched implicitly.

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

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10303)