StatusCode: remove deprecated checking code

Remove the deprecated StatusCode "checking" code. This code is non-functional and marked with a deprecation warning since %v36r0 (see !989 (merged)). Also delete deprecated implicit type conversion constructors.

This only cleans up the checking code in StatusCode. In a separate MR (!1310 (merged)) we can try deleting the useless StatusCodeSvc itself (we no longer have any reference in ATLAS, but not sure about LHCb).