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

Fix #28231, and remove some unused code. The `db.HasEngine` doesn't seem
useful because the db engine is always initialized before web route.

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.