Avoid cycle-redirecting user/login page (#28636) (#28658)

Backport #28636

Fix #28231, and remove some unused code.

models/auth/source.go

@@ -275,9 +275,6 @@ func ActiveSources(tp Type) ([]*Source, error) {
 // IsSSPIEnabled returns true if there is at least one activated login
 // source of type LoginSSPI
 func IsSSPIEnabled() bool {
-	if !db.HasEngine {
-		return false
-	}
 	sources, err := ActiveSources(SSPI)
 	if err != nil {
 		log.Error("ActiveSources: %v", err)

models/db/engine.go

@@ -27,9 +27,6 @@ var (
 	x         *xorm.Engine
 	tables    []any
 	initFuncs []func() error
-
-	// HasEngine specifies if we have a xorm.Engine
-	HasEngine bool
 )
 
 // Engine represents a xorm engine or session.