There is absolutely no NPC that trains this. Yes, I've searched high and low and spent quite a considerable amount of time gong from town to town.  Even the Theif Guildmaster at the New Player - Lockpicking/Thief Training room trains every "theiving skills" except remove trap.  Also those lock picking boxes doesn't give us the ability to remove trap to skill raise. I tried it with raising a few points on the skill with the skill ball.  Was this intentionally removed from this shard or is it a bug?   I paged about it and Joker responded that it may be something that was overlooked and the only solution for now is to use my skill ball, which I am frugally saving for some other harder skills to raise. This is quite annoying to say the least to thief/treasure hunters community. Joker said this issue can be fixed but not high on the list.  What a bummer. 

  Well, you didn't look everywhere. Tinkers make the traps, so they show you how to disarm them. I am 100% certain the Britian tinker will train you in remove trap to near 30 skill.

You're right. One would think it would be on the Thieves NPC/Guildmaster list since its part of the thieving to remove traps from chests.  Also the the lockpick boxes at the training house don't have disarm trap training capability.  This would be much beneficial to include it for training.

Train it at the same time you train tinkering and carpentry. Short guide at the end of this page: http://www.uoguide.com/Remove_Trap

Though I do agree a set of automatically re-trapping boxes would be a nice addition to the re-locking boxes.