chore(data/fintype): drop a decidable_pred assumption (#14971)

OTOH, now the proof depends on `classical.choice`.

src/data/fintype/basic.lean

@@ -1004,8 +1004,9 @@ begin
 end
 
 /-- If the subtype of all-but-one elements is a `fintype` then the type itself is a `fintype`. -/
-def fintype_of_fintype_ne (a : α) [decidable_pred (= a)] (h : fintype {b // b ≠ a}) : fintype α :=
-fintype.of_equiv _ $ equiv.sum_compl (= a)
+def fintype_of_fintype_ne (a : α) (h : fintype {b // b ≠ a}) : fintype α :=
+fintype.of_bijective (sum.elim (coe : {b // b = a} → α) (coe : {b // b ≠ a} → α)) $
+  by { classical, exact (equiv.sum_compl (= a)).bijective }
 
 section finset