Remove Drain and Oil Rig

This MR targets the new pending-release branch, not master.

Removes Drain and Oil Rig (currently hidden from 0.8.5 menus).
This MR removes all associated files, including the Oil Rig texture set (texures/map_oilrig) and its shaders. I'm not sure about this because there might be 3rd party maps around that make use of this texture set as if it were a core set (they shouldn't).
However it appears to have been done this way for previously removed official maps: https://forums.xonotic.org/showthread.php?tid=7258

Opinions on this?

Glowplant was making use of sound/map_drain/spark.ogg, so I moved it to sound/map_glowplant/spark.ogg instead.
I used grep to check that no other official map uses textures/models/sounds from Drain or Oil Rig.