`compas_rhino.utilities.delete_layers` deletes whole tree when given dict as input.

## Describe the bug
Unexpected behavior when using a dict as parameter for `compas_rhino.utilities.delete_layer`.

(I might be misunderstanding though, let me know if so and I can probably updated docstring)

## To Reproduce

### Environment
Rhino Python Editor

### Sample script
```
from compas_rhino.utilities import create_layers, delete_layers
layers = {'COMPAS': {'layers': {'Datastructures': {'layers': {'Mesh': {}, 'Network': {}}}}}, "SAPMOC": {}}
create_layers(layers)
delete_layers({'COMPAS': {'layers': {'Datastructures': {'layers': {'Network': {}}}}}})
```

### Result


## Expected result


### Other environment details
**Desktop (please complete the following information):**
- OS: Windows 10
- Python version: 2.7.8 (IronPython 2.7.8 (2.7.8.0) on .NET 4.0.30319.42000 (64-bit))
- Python package manager: conda