Remove unused multiply & divide methods from Complex Number(#2852)

This is because we haven't implemented the tests for multiplying or dividing a complex number with a real number (according to comments in tests.toml).

exercises/practice/complex-numbers/.meta/src/reference/java/ComplexNumber.java

@@ -1,7 +1,6 @@
 final class ComplexNumber {
 
     private final double real;
-
     private final double imaginary;
 
     ComplexNumber(double real, double imaginary) {
@@ -32,7 +31,10 @@ ComplexNumber subtract(ComplexNumber other) {
     }
 
     ComplexNumber divide(ComplexNumber other) {
-        return this.multiply(other.conjugate()).divide(Math.pow(other.abs(), 2));
+        double divisor = Math.pow(other.real, 2) + Math.pow(other.imaginary, 2);
+        return new ComplexNumber(
+            (real * other.real + imaginary * other.imaginary) / divisor,
+            (imaginary * other.real - real * other.imaginary) / divisor);
     }
 
     double abs() {
@@ -44,14 +46,7 @@ ComplexNumber conjugate() {
     }
 
     ComplexNumber exponentialOf() {
-        return new ComplexNumber(Math.cos(imaginary), Math.sin(imaginary)).multiply(Math.exp(real));
+        return new ComplexNumber(Math.exp(real) * Math.cos(imaginary), Math.exp(real) * Math.sin(imaginary));
     }
 
-    private ComplexNumber divide(double factor) {
-        return new ComplexNumber(real / factor, imaginary / factor);
-    }
-
-    private ComplexNumber multiply(double factor) {
-        return new ComplexNumber(factor * real, factor * imaginary);
-    }
 }

exercises/practice/complex-numbers/src/main/java/ComplexNumber.java

@@ -28,24 +28,15 @@ ComplexNumber multiply(ComplexNumber other) {
         throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
     }
 
-    ComplexNumber multiply(double factor) {
-        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
-    }
-
     ComplexNumber divide(ComplexNumber other) {
         throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
     }
 
-    ComplexNumber divide(double divisor) {
-        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
-    }
-
     ComplexNumber conjugate() {
         throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
     }
 
     ComplexNumber exponentialOf() {
         throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
     }
-
 }