refactor: remove unused Random.categorySlug() method

Should be in 399cbbd commit.

Part of #1668

Author: php-coder

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

@@ -156,10 +156,6 @@ public static String categoryName() {
 		return name;
 	}
 
-	public static String categorySlug() {
-		return SlugUtils.slugify(categoryName());
-	}
-	
 	public static String countryName() {
 		String name = between(
 				CountryValidation.NAME_MIN_LENGTH,