[calendar] chore: remove requiresVersion: "2.1.0" (#3932)

This is just to reduce a little noise in the dev console. The following
line will not appear with this change:

```shell
module.js:480 Check MagicMirror² version for module 'calendar' - Minimum version:  2.1.0 - Current version: 2.34.0-
```

Since version 2.1.0 is so old, we can surely throw it out without
concern.

modules/default/calendar/calendar.js

@@ -68,8 +68,6 @@ Module.register("calendar", {
 		updateOnFetch: true
 	},
 
-	requiresVersion: "2.1.0",
-
 	// Define required scripts.
 	getStyles () {
 		return ["calendar.css", "font-awesome.css"];