delete 3rdparty folder, it prevents auto discovery in parent folder

Currently, when I checkout master an empty 3rdparty folder is created. (I think at least it just overwrote my link). The should be removed, becaus it prevents the 3rdparty discovery logic in https://github.com/owncloud/core/blob/master/lib/base.php#L156 to use the 3rdparty folder in the parent directory.

I admit not having read up on how 3rdparty is actually resolved, so I constatly ran into issues having to create a link or adjusting the config. This PR should make it just work automagically again. Dunno when it was broken...

@bartv2 @Raydiation @icewind1991 @DeepDiver1975 please :+1: 

cc @karlitschek I hope this does not interfere with any deploy script you may have...