Spring clean weapon_medkit.lua

- Simplify `need` variables
- Implement DRY in the Primary/Secondary firing, I hope line 48 works as I haven't tested any of this out.
- I originally intended to fix the line spacing but half-way (after fixing the spacing in the GitHub web editor) through I switched editors and found that the line spacing was mucked up. I decided to leave it alone because it'll show awkwardly on different editors anyway.