difference-of-squares: remove starter impl

exercises/difference-of-squares/src/main/java/DifferenceOfSquaresCalculator.java

-final class DifferenceOfSquaresCalculator {
-
-    int computeSquareOfSumTo(final int input) {
-        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
-    }
-
-    int computeSumOfSquaresTo(final int input) {
-        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
-    }
-
-    int computeDifferenceOfSquares(final int input) {
-      throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
-    }
-
-}