make timing consistent across exercises

exercises/exerciseA/heavy_computation.py

@@ -19,8 +19,7 @@ print(f"We are executed with OMP_NUM_THREADS={threads} for {n=}")
 # Measure the time required for matrix multiplication
 start_time = time.time()
 y = x @ x  # The heavy compute
-stop_time = time.time()
-elapsed_time = stop_time - start_time
+elapsed_time = time.time() - start_time
 
 print(f'Time used for matrix multiplication: {elapsed_time:.2f} s')