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.

tests/integration/markup_external_test.go

@@ -19,7 +19,7 @@ import (
 
 func TestExternalMarkupRenderer(t *testing.T) {
 	defer tests.PrepareTestEnv(t)()
-	if !setting.Database.UseSQLite3 {
+	if !setting.Database.Type.IsSQLite3() {
 		t.Skip()
 		return
 	}