Remove deprecated Array::split_at function and update documentation

Author: bobzhang

builtin/array.mbt

@@ -728,6 +728,7 @@ pub fn[T] Array::rev(self : Array[T]) -> Array[T] {
 ///   assert_eq(v2, [4, 5])
 /// ```
 #deprecated("Use ArrayView instead, e.g, (a[:index], a[index:])")
+#doc(hidden)
 pub fn[T] Array::split_at(self : Array[T], index : Int) -> (Array[T], Array[T]) {
   if index < 0 || index > self.length() {
     let len = self.length()

builtin/pkg.generated.mbti

@@ -144,8 +144,6 @@ fn[T] Array::search_by(Self[T], (T) -> Bool) -> Int?
 fn[T] Array::set(Self[T], Int, T) -> Unit
 fn[T] Array::shrink_to_fit(Self[T]) -> Unit
 fn[T] Array::split(Self[T], (T) -> Bool raise?) -> Self[Self[T]] raise?
-#deprecated
-fn[T] Array::split_at(Self[T], Int) -> (Self[T], Self[T])
 fn[T : Eq] Array::starts_with(Self[T], Self[T]) -> Bool
 fn[T : Eq] Array::strip_prefix(Self[T], Self[T]) -> Self[T]?
 fn[T : Eq] Array::strip_suffix(Self[T], Self[T]) -> Self[T]?