Remove redundent function in Backtracking Sudoku (#4499)

* Remove redundent function

After reviewing this code, I've noticed that the `is_completed` function is a redundant operation.
Increasing the number of loops required for each step of the sudoku solver. 
This should remove n² operations where n is the width of the grid.

* Update sudoku.py

Remove additional newline

backtracking/sudoku.py

@@ -59,27 +59,6 @@ def is_safe(grid: Matrix, row: int, column: int, n: int) -> bool:
     return True
 
 
-def is_completed(grid: Matrix) -> bool:
-    """
-    This function checks if the puzzle is completed or not.
-    it is completed when all the cells are assigned with a non-zero number.
-
-    >>> is_completed([[0]])
-    False
-    >>> is_completed([[1]])
-    True
-    >>> is_completed([[1, 2], [0, 4]])
-    False
-    >>> is_completed([[1, 2], [3, 4]])
-    True
-    >>> is_completed(initial_grid)
-    False
-    >>> is_completed(no_solution)
-    False
-    """
-    return all(all(cell != 0 for cell in row) for row in grid)
-
-
 def find_empty_location(grid: Matrix) -> Optional[Tuple[int, int]]:
     """
     This function finds an empty location so that we can assign a number
@@ -111,12 +90,7 @@ def sudoku(grid: Matrix) -> Optional[Matrix]:
      >>> sudoku(no_solution) is None
      True
     """
-
+    if location := find_empty_location(grid):
-    if is_completed(grid):
-        return grid
-
-    location = find_empty_location(grid)
-    if location is not None:
         row, column = location
     else:
         # If the location is ``None``, then the grid is solved.