Refactor setting.Database.UseXXX to methods (#23354)

Replace #23350.

Refactor `setting.Database.UseMySQL` to
`setting.Database.Type.IsMySQL()`.

To avoid mismatching between `Type` and `UseXXX`.

This refactor can fix the bug mentioned in #23350, so it should be
backported.

models/migrations/v1_13/v140.go

@@ -41,7 +41,7 @@ func FixLanguageStatsToSaveSize(x *xorm.Engine) error {
 
 	// Delete language stat statuses
 	truncExpr := "TRUNCATE TABLE"
-	if setting.Database.UseSQLite3 {
+	if setting.Database.Type.IsSQLite3() {
 		truncExpr = "DELETE FROM"
 	}