chore: remove unnecessary partial in Lean.Expr (#8464)

The termination prover has gotten stronger since these definitions were
written, and now they can be proved terminating automatically. (One
definition had to be changed slightly because it wasn't actually
terminating before.)

Author: digama0

src/Lean/Expr.lean

@@ -750,7 +750,7 @@ def mkStrLit (s : String) : Expr :=
 def mkAppN (f : Expr) (args : Array Expr) : Expr :=
   args.foldl mkApp f
 
-private partial def mkAppRangeAux (n : Nat) (args : Array Expr) (i : Nat) (e : Expr) : Expr :=
+private def mkAppRangeAux (n : Nat) (args : Array Expr) (i : Nat) (e : Expr) : Expr :=
   if i < n then mkAppRangeAux n args (i+1) (mkApp e args[i]!) else e
 
 /-- `mkAppRange f i j #[a_1, ..., a_i, ..., a_j, ... ]` ==> the expression `f a_i ... a_{j-1}` -/
@@ -1502,8 +1502,8 @@ abbrev PersistentExprStructMap (α : Type) := PHashMap ExprStructEq α
 
 namespace Expr
 
-private partial def mkAppRevRangeAux (revArgs : Array Expr) (start : Nat) (b : Expr) (i : Nat) : Expr :=
-  if i == start then b
+private def mkAppRevRangeAux (revArgs : Array Expr) (start : Nat) (b : Expr) (i : Nat) : Expr :=
+  if i ≤ start then b
   else
     let i := i - 1
     mkAppRevRangeAux revArgs start (mkApp b revArgs[i]!) i
@@ -1880,7 +1880,7 @@ def updateFn : Expr → Expr → Expr
 /--
 Eta reduction. If `e` is of the form `(fun x => f x)`, then return `f`.
 -/
-partial def eta (e : Expr) : Expr :=
+def eta (e : Expr) : Expr :=
   match e with
   | Expr.lam _ d b _ =>
     let b' := b.eta