Fixed content_abm aliases

Since commit 5a13c49492 when I'm in a new area of the world that hasn't yet been generated the debug log gets flooded with the following error:

```
21:49:43: ERROR[ServerThread]: Map::setNode(): Not allowing to place CONTENT_IGNORE
while trying to replace "default:dirt_with_grass" at (729,1,-219) (block (45,0,-14))
21:49:43: INFO[ServerThread]: Debug stacks:
21:49:43: INFO[ServerThread]: DEBUG STACK FOR THREAD 4688: 
21:49:43: INFO[ServerThread]: #0  MeshUpdateThread::Thread
21:49:43: INFO[ServerThread]: DEBUG STACK FOR THREAD 5304: 
21:49:43: INFO[ServerThread]: #0  ServerThread::Thread
21:49:43: INFO[ServerThread]: #1  Server::AsyncRunStep
21:49:43: INFO[ServerThread]: #2  ServerEnvironment::step
21:49:43: INFO[ServerThread]: DEBUG STACK FOR THREAD 7392: 
21:49:43: INFO[ServerThread]: #0  main
21:49:43: INFO[ServerThread]: #1  ClientMap::renderMap
21:49:43: INFO[ServerThread]: DEBUG STACK FOR THREAD 8936: 
21:49:43: INFO[ServerThread]: #0  EmergeThread::Thread
```

This commit fixes that, However, I'm not 100% sure if this is the correct fix as I'm not really sure why the aliases in the above mentioned commit were changed in the first place. 

**So if someone could please double check this is fine or indeed the correct way to fix the issue before pushing upstream.**