Remove diff function from TreeSet implementation

Removed the diff function for TreeSet instances.

Author: srghma

src/Std/Data/TreeSet/Basic.lean

@@ -519,14 +519,6 @@ def inter (t₁ t₂ : TreeSet α cmp) : TreeSet α cmp :=
 
 instance : Inter (TreeSet α cmp) := ⟨inter⟩
 
-/--
-Computes the difference of the given tree sets.
-
-This function always iterates through the smaller set.
--/
-def diff (t₁ t₂ : TreeSet α cmp) : TreeSet α cmp :=
-  ⟨TreeMap.diff t₁.inner t₂.inner⟩
-
 instance : SDiff (TreeSet α cmp) := ⟨diff⟩
 
 /--