1. Different difficulty settings allow for a significantly different gaming experience, for the same mission, as mobs (and traps) have will different characteristics.
2. Dungeon scaling for team size allows teams of various different numbers of players to have a roughly equivalent level of challenge, for any given level of difficulty selected -- at least, the level of difficulty across different team sizes will generally be more equal, when scaling is implemented moderately well.
These are not the same things.
It is not possible to remove either dungeon scaling or difficulty level and still give players the same level of flexibility to choose the kind of gaming experience they prefer. This would also apply to teaming situations, incidentally, so consider carefully before arguing that dungeon scaling and difficulty settings are "redundant", and that one or the other should be removed from the game.
Dungeon scaling means that a team leader can put up a LFM and specify a difficulty setting in advance, then let team size settle wherever it settles, and go adventuring. The team leader can pre-specify the difficulty, and the dungeon will adjust to team size, rather than putting a team leader in the position of trying to make a judgment about the appropriate difficulty, based on team composition. Also, if you're thinking of joining a team, knowing the anticipated level of difficulty in advance can be helpful (at least for more casual players), as a player may feel they're ready for normal, but not for elite.