Revert "Refactored ReverseStringUsingStack utility for reversing strings using stack"

This reverts commit e091e22.

Author: navaneedan07

src/main/java/com/thealgorithms/strings/ReverseString.java

@@ -66,7 +66,6 @@ public static String reverse3(String string) {
      * and then pops them off to create the reversed string.
      */
     public static String reverse4(String str) {
-        // Check if the input string is null
         if (str == null) {
             throw new IllegalArgumentException("Input string cannot be null");
         }
@@ -86,4 +85,4 @@ public static String reverse4(String str) {
         }
         return reversedString.toString();
     }
-}
+}