OMG how weird. I've just got the same error when trying to delete a plugin :O
maybe this is a problem with myBB 1.6.2 although I can't see how...
Did you make any edits to the plugin file? I'm using 1.6.2 on localhost, and was able to activate and deactivate the plugin no problem.
You've to delete the icons column from mybb_forums manually.
The problem is that you're trying to remove a database column that doesn't exist. Add the column back manually before trying to remove the plugin.