Event code fixes.

- Previously base event chance would get applied twice, so instead of e.g. 0.03 chance, a 0.03^2 chance
- Removed unused code
Merge branch 'bleeding-edge-freeze' of github.com:Baystation12/Baystation12 into feature

Conflicts:
	code/game/gamemodes/events.dm
Merge pull request #2235 from CIB/feature

Event code fixes.