refactor: remove unused Random.countrySlug() method

Should be in 95bfe50 commit.

Part of #1668

Author: php-coder

src/test/java/ru/mystamps/web/tests/Random.java

@@ -172,10 +172,6 @@ public static String countryName() {
 		return name;
 	}
 
-	public static String countrySlug() {
-		return SlugUtils.slugify(countryName());
-	}
-	
 	public static String participantName() {
 		return between(
 			ParticipantValidation.NAME_MIN_LENGTH,