Remove kill count from core

Kill count has been tracked by Lua for a year (see libs/Characters.lua and modules/StatsTracking), and with the new-ui info view (#1633) the value from core is no longer used anywhere.

The translations for the combat ratings and kill messages have been removed from the core translation files. For languages which already have partial translations in data/ui/Languages.lua, I have moved the combat rating translations there.

If this is merged, #678 can be closed.

For now I have retained save file compatibility by writing/ignoring a couple of dud values; I'm a bit fed up with breaking saves all the time.