Study what's causing seg faults with map layers to drop custom workaround to clean those

[LKajan]

Currently we have two alternative ways to make sure map layers are cleaned completly to avoid seg faults. One is automatically method to run the cleanin process layers returned by fixuters that have a specific keyword in their names and another is using @clean_qgis_layer decorator with the fixture.

The "cleaning" process is to add the layer to the project and immediately remove it from the project. This prevents segmentation faults but why?

Ideally we wouldn't need any extra functionality to manage the layer cleaning.

refs: #64, #45