Deleting an upload does not work

[StefanVanDyck]

Delete an uploaded layer from the admin interface on version 2.1.5 does not work.
It seems like the logic was disabled for some reason.

I created a PR with some simple changes that seem to do the trick:
#245

[StefanVanDyck]

Yeah that pr deletes stuff allright, but is not working correctly, I am probably misunderstanding what is going on.
Although I have a suspicion layer_id and raw_id are not always used correctly everywhere.

Not sure if this is expected behavior, but have a bunch of folders being created in the data folder for actual layers and not just uploads

[StefanVanDyck]

Created a new simpler and hopefully better PR that seems to allow deleting uploads again.
#248