Error when using EagerLoad on a relationship

This was supposed to be fixed in https://github.com/json-api-dotnet/JsonApiDotNetCore/pull/1000 and tested with https://github.com/json-api-dotnet/JsonApiDotNetCore/pull/990. But as it turns out, the fix wasn't merged correctly.

The unconditional line below the fix:
```c#
propertySelectors[propertySelector.Property] = propertySelector;
```
should be removed.