Remove unused graph_impl function.

Author: slawekptak

sycl/source/detail/graph_impl.hpp

@@ -919,10 +919,6 @@ class graph_impl : public std::enable_shared_from_this<graph_impl> {
   /// @return Device associated with graph.
   sycl::device getDevice() const { return MDevice; }
 
-  const DeviceImplPtr &getDeviceImplPtr() const {
-    return getSyclObjImpl(MDevice);
-  }
-
   /// List of root nodes.
   std::set<std::weak_ptr<node_impl>, std::owner_less<std::weak_ptr<node_impl>>>
       MRoots;