On the svn-auth-x509 branch, remove the unused error code.

* subversion/include/svn_error_codes.h
  (SVN_ERR_X509_CERT_UNKNOWN_SIG_ALG): Remove, we'll never use this see
    r1605645 for reason why.
  (SVN_ERR_X509_CERT_UNKNOWN_PK_ALG, SVN_ERR_X509_CERT_SIG_MISMATCH,
   SVN_ERR_X509_CERT_VERIFY_FAILED): Renumber.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/svn-auth-x509@1605786 13f79535-47bb-0310-9956-ffa450edef68