cleanup(samlsp): remove deprecated AttributeMapper

The AttributeMapper was replaced by the unified ClaimTransformer system
(pkg/credential/transformer.go) which handles both SAML and OIDC RP
flows identically. The mapper operated on SAML-specific map[string][]string,
output flat claims (no dot-notation nesting), and had no remaining references
outside its own file.

Closes SUNET#298

Author: leifj