Removed cpos = ~cpos constructions for closing a well

This is a fallout from https://github.com/OPM/opm-core/pull/507

Closing wells was performed by:
```C++
cpos = ~cpos;
set_current_control( );
```

This fails hard when a well has been shut and has no valid controls. With this PR the `well_controls_shut_well()` is used.