Not semantics for semantics' sake:
I reckon it's right on the line between field mod and "just" pilot/crew choice. You are definitely "modifying" if e.g. you add some non standard guns (not even available in e.g. for germany the factory provided rüstsätze packages) or some innovative mod like taking propeller from a different plane, that happens to work even better on your plane. Or souping up your engine, or punching a hole in fuselage to fix an overheat problem that the rest of that model aircraft in the whole airforce are still plagued with, etc. All these are clearly mods.
But just omitting something, that's only borderline field mod IMO. Especially if removing it doesn't require changing the rest of the plane as a consequence. Many 410s removed MG17s when they used some of the standard rüstsätze, e.g. 6x 20mm. And it's understandable - not much sense for such a small firepower bonus at the cost of the extra weight, esp. if the 6x20mm was already heavy on its own.
Running with this same example, 8x 20mm as at least one (as far as there's reports of anyway) pilot used instead of the non-field mod BK5 50mm cannon, that's clearly enough a field mod. Not so arguable, even if all he did was combine two ordinary rüstsätze: 4x20mm in the bomb bay, and 2x20mm in a belly pack like on our 110G.
Another arguable case of field mod, as far as AH's criteria for what a field mod is: some 410 crews added a pair of 7.9mm guns, apparently where no guns are on any factory configuration (directly above bomb bay guns, below cockpit guns, it's not clear which compartment they're in) - they had to punch holes for gun ports. That's much more like a field mod.
But just removing things that don't require any reworking of the aircraft because of it.. I dunno, that's not in the same league as actual field mods where you've got a different plane as a result. Ideally I'd make it a perk loadout myself. Because the way AH deals with it is to ask - what was the standard configuration that most crews adopted? What would you have most commonly run into at the time?
It's not only whether something's a field mod.