[SYCL][NFC] Remove dead code and update a comment (#1987)

Author: bader

sycl/include/CL/sycl/detail/cg.hpp

@@ -34,16 +34,14 @@
 __SYCL_INLINE_NAMESPACE(cl) {
 namespace sycl {
 
-// Forward declaration
+// Forward declarations
 class queue;
 namespace detail {
 class queue_impl;
 } // namespace detail
 
 namespace detail {
 
-using namespace cl;
-
 class stream_impl;
 /// Base class for all types of command groups.
 class CG {