Next patch (5.0, Karak release) will have a menu setting to toggle melee weapon attack VFX on/off. :)
EDIT: See my self-reply below.
Unfortunately during today's testing we found out that flails and a mace skin (Feeble Mace) do not benefit from this setting. As a workaround, anyone eager to remove the fire can do so by setting their Particle Quality (under Video settings) to Lowest.
[ Note: this setting is the reason why this went by under the radar on the first place. :( ]
I'm really sorry about this. I know this isn't ideal but we're working hard on making sure this gets into a hotfix swiftly after release.
Cheers// Calavera