Deregistering a user-defined datatype. Register multiple dtypes to same ctype.

The usecase is that different devices have different alignments for a particular c struct. This raises the need for different numpy datatypes to be registered as same ctype which requires a provision where different numpy datatypes to registered as a particular ctype depending on the device that we working with at that time.

For example, the following simple struct
```
typedef struct {
int f1;
long f2;
} temp;
```
results in these two different datatypes when matched on two different `dtype({'names':['f1','f2'], 'formats':['