Remove non-essential null-checking from gsl::not_null

[dmitrykobets-msft]

We are thinking of revising the gsl::not_null threat-model to reserve all nullness checking to the constructor, and to trust all created instances post-construction:

not_null<int*> a = some_pointer; // check that some_pointer is not null in a's constructor
int* b = a.get(); // no need to check that `a.ptr_` is not null; it's already been checked in a's constructor
not_null<int*> other = a; // no need to check that `a.ptr_` is not null; it's already been checked in a's constructor.  
                          // transitively, `other._ptr_` will now also be trusted

This change will remove many existing runtime checks that occur inside gsl::not_null, notably the one inside not_null::get(), and will overall simplify the design of the gsl::not_null type.

[KindDragon]

I think the post at the link is relevant to this issue #930 (comment)