Don't follow legacy path if ctx->pctx is set

Don't follow legacy path if ctx->pctx is set

EVP_DigestInit_ex forced following of the legacy path if ctx->pctx is
set (meaning we've actually been called via EVP_DigestSignInit_ex).

There is some code in the legacy path that calls the
EVP_PKEY_CTRL_DIGESTINIT ctrl on the pctx. Not going down the legacy path
if ctx->pctx is set means that ctrl message will neve get sent. However,
it turns out that all algs that understand that ctrl also set the
EVP_MD_CTX_FLAG_NO_INIT flag which forces legacy anyway. Therefore the
ctx->pctx check is not required and can be removed.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10082)