SAML: Do not SAML SLO if user is not SAML authenticated (#53418)

* Only SLO user if the user is using SAML

* only one source of truth for auth module info

* ensure SAML is also enabled and not only SLO

* move auth module naming to auth module login package

* use constants in other previously unused spots

pkg/models/user_auth.go

@@ -9,11 +9,6 @@ import (
 	"golang.org/x/oauth2"
 )
 
-const (
-	AuthModuleLDAP  = "ldap"
-	AuthModuleProxy = "authproxy"
-)
-
 type UserAuth struct {
 	Id                int64
 	UserId            int64