flamethrower_alt_fire_oil: Drop supercede and use airblast functionality flags

[nosoop]

FlaminSarge pointed out to me that there are a number of airblast_* attributes that control the functionality of airblast.

The idea would be to modify the plugin so it doesn't handle airblast cost / refire; server operators can use the existing flags to remove the knockback mechanic on the flamethrower (or even keep it alongside the oil spill if they choose).