Too late, I already destructed it this morning. But I'm not sure if a core removal tool would've helped, because I tried to rotate it with a pliers and it didn't budge. Then I tried to break the stem from the tyre, but it was too slow and risked damaging the rim, so at the end I just twisted the insert of the valve (the small thing that you would push in to release air) and broke it. Then I was able to push a small screwdriver inside that made enough airflow to deflate it. I guess I was lucky, because it could've been totally clogged, but at least now I would theoretically reuse the inner tube after cleaning the valve and replacing the core. But I don't think I'll do that, because not much sealant has remained in it, it has ~ half a dozen thorns in it. So I just replaced it with another inner tube (that also has sealant in it, that already causes problems, because even to inflate it slightly, just to have enough air so it doesn't fall out of the tyre, I had to fight it. It already started to clog, before I even put it on the rim...

BTW "luckily" the inside of the tyre was also wet, so I cleaned it, which was fortunate, because I found 4 thorns in the inside , which might have been able to puncture the new inner tube pretty soon. (Usually I remember to check the inside of the tyre an remove the thorn after patching a flat inner tube, but this time I didn't think / remember that I had thorns in it, as my whole problem was that there was too much air in it

Which also made me think, that massy as it is, maybe this is another advantage of tubeless: the sealant seals the small holes around the thorns, and even though the thorn remains in the tyre, it won't probably cause another flat. On the other hand when using inner tubes with sealant, the thorn remains in the tyre, poking the inner tube again and again ...