style: enable RedundantModifier in checkstyle (#5140)

src/main/java/com/thealgorithms/maths/FFT.java

@@ -27,7 +27,7 @@ public class FFT {
         /**
          * Default Constructor. Creates the complex number 0.
          */
-        public Complex() {
+        Complex() {
             real = 0;
             img = 0;
         }
@@ -38,7 +38,7 @@ public class FFT {
          * @param r The real part of the number.
          * @param i The imaginary part of the number.
          */
-        public Complex(double r, double i) {
+        Complex(double r, double i) {
             real = r;
             img = i;
         }