Remove requirement on DefaultScope

We decided that this requirement doesn't actually help implementations.

Author: gmlueck

adoc/chapters/programming_interface.adoc

@@ -20340,10 +20340,6 @@ scope for the atomic operations.
 Most member functions also provide an optional parameter that allows the
 application to override this default.
 
-All accesses to an object referenced by an [code]#sycl::atomic_ref# must
-exclusively occur through instances of an [code]#sycl::atomic_ref# with the same
-[code]#DefaultScope#.
-
 The [code]#sycl::atomic_ref# class also has a template parameter
 [code]#AddressSpace#, which allows the application to make an assertion about
 the address space of the object of type [code]#T# that it references.