Consider codechecker_max_warnings should instead just be codechecker_warnings

[brendanheywood]

Lets say codechecker_max_warnings is set to 10 and you make some change and now there are actually only 9 warnings. In this case I think it should throw an error and ask you to change the number from 10 to 9 so it ratchets down. So the only valid number of warnings is the exact number rather than a max