Use IsProd instead of testing if it's equal. (#14336)

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: zeripath <art27@cantab.net>

modules/setting/setting.go

@@ -376,7 +376,6 @@ var (
 	CustomConf    string
 	PIDFile       = "/run/gitea.pid"
 	WritePIDFile  bool
-	ProdMode      bool
 	RunMode       string
 	RunUser       string
 	IsWindows     bool
@@ -388,6 +387,11 @@ var (
 	UILocation = time.Local
 )
 
+// IsProd if it's a production mode
+func IsProd() bool {
+	return strings.EqualFold(RunMode, "prod")
+}
+
 func getAppPath() (string, error) {
 	var appPath string
 	var err error