fix warnings

app/src/main/java/sudoku/Main.java

@@ -9,12 +9,9 @@ import sudoku.io.ConsoleInterface;
 import sudoku.io.SudokuPrinter;
 import sudoku.io.SudokuSerializer;
 import sudoku.solver.RandomSolver;
-import sudoku.solver.Solver;
 import sudoku.structure.MultiDoku;
 import sudoku.structure.SudokuFactory;
 
-import java.util.Random;
-
 public class Main {
     public String getGreeting() {
         return "Hello World!";