remove_subtag() may be broken

[8573]

From elsewhere:

Vultraz: I now tested it [[remove_object]] with this: http://pastebin.com/iwLWGWVf it does not appear to remove anything
Vultraz: is that a bug?
8573: I think so.
8573: I just realized, there is no unit filter in that test.
Vultraz: we set it up so NX provided a default filter
8573: We did?
8573: I forgot.
8573: I’ve looked at lp8 modifications.lua, and I can’t see how it could fail to remove the object when skip_effects is yes, unless (a) lp8.remove_object [I meant remove_subtag] is broken or (b) you ignored another error message.
8573: I think lp8.remove_object may indeed be broken — that would explain the new_ability removal bug as well. Can you confirm?

I am filing this issue as a placeholder for the potential nonfunctionality of wml/remove_subtag as mentioned above.

@Vultraz: When you have time, please run this test:

-- Use whatever filter you like, as long as it
-- finds a unit that has at least one attack.
lp8.import 'wml'
local unit = wesnoth.get_units {…}[1].__cfg
local before_attacks = get_subtags(unit, 'attack')
remove_subtag(unit, 'attack')
local after_attacks = get_subtags(unit, 'attack')
assert(#before_attacks >= #after_attacks)

[Vultraz]

I cannot remember if I ever got around to running this, but trying now, it fails with "get_stubtags" being a nil value. I'm not sure if it's an lp8 import issue, or the way we set up lp8 to load.