All the signature algorithm logic depends on X509_ALGOR. This also
removes the X509_ALGOR-based EVP functions which are no longer used
externally. I think those APIs were a mistake on my part. The use in
Chromium was unnecessary (and has since been removed anyway). The new
X.509 stack will want to process the signatureAlgorithm itself to be
able to enforce policies on it.

This also moves the RSA_PSS_PARAMS bits to crypto/x509 from crypto/rsa.
That struct is also tied to crypto/x509. Any new RSA-PSS code would
have to use something else anyway.

BUG=499653

Change-Id: I6c4b4573b2800a2e0f863d35df94d048864b7c41
Reviewed-on: https://boringssl-review.googlesource.com/7025
Reviewed-by: Adam Langley <agl@google.com>