Fix warning by not calling now-empty and deprecated retain().

```
OpenMesh/Core/Utils/PropertyManager.hh:393:16: warning: 'retain' is deprecated [-Wdeprecated-declarations]
  393 |             pm.retain();
      |                ^
OpenMesh/Core/Utils/PropertyManager.hh:325:9: note: 'retain' has been explicitly marked deprecated here
  325 |         OM_DEPRECATED("retain no longer has any effect. Instead, named properties are always retained, while unnamed ones are not.")
```

src/OpenMesh/Core/Utils/PropertyManager.hh

@@ -390,7 +390,6 @@ class PropertyManager {
                 const PROP_VALUE &init_value) {
             const bool exists = propertyExists(mesh, propname);
             PropertyManager pm(mesh, propname, exists);
-            pm.retain();
             if (!exists)
                 pm.set_range(begin, end, init_value);
             return std::move(pm);