Deleted problem's solution is assumed to be a new problem's solution

[ngsankha]

If the admin adds a problem and the user submits a solution to it and it is correct he gets a solved tag. But later when the admin removes the old problem and adds a new one, then the old solution is taken to be the solution of the new problem and if the old solution is correct then the new problem is also marked correct.

[ngsankha]

The solution to this problem seems to be in deleting the already submitted solutions from the database because they are useless now. Fixing it accordingly.