Disabled the game using set background by making all instances of it use a define, which can be changed in code/_compile_options.dm
Testing has revealed that it reduces the sluggishness of the game, though it will spike from lag when the singularity is loose. Thanks to ChuckTheSheep for suggesting it.

Server owners who want to keep set background enabled can do so by changing the define.
Merge pull request #2305 from Giacom/removed_set_background

Disabled the game using "set background" by making all instances of it use a define