Remove exception from empty_allocator::deallocate()

The data copy will call deallocate no matter what, so be graceful.
We will still catch cases where we try to allocate through
the empty allocator.

Signed-off-by: Joseph Schuchart <[email protected]>

ttg/ttg/parsec/buffer.h

@@ -35,7 +35,7 @@ namespace detail {
     }
 
     void deallocate(value_type* ptr, std::size_t size) {
-      throw std::runtime_error("Deallocate on empty allocator!");
+      /* nothing to be done; will be called from ~data_copy_type() */
     }
   };