Remove unused Index(Mut) impls on AST types (#11903)

# Description
Both `Block` and `Pipeline` had `Index`/`IndexMut` implementations to
access their elements, that are currently unused.
Explicit helpers or iteration would generally be preferred anyways but
in the current state the inner containers are `pub` and are liberally
used. (Sometimes with potentially panicking indexing or also iteration)

As it is potentially unclear what the meaning of the element from a
block or pipeline queried by a usize is, let's remove it entirely until
we come up with a better API.

# User-Facing Changes
None

Plugin authors shouldn't dig into AST internals

crates/nu-protocol/src/ast/block.rs

@@ -1,7 +1,6 @@
 use super::Pipeline;
 use crate::{ast::PipelineElement, Signature, Span, Type, VarId};
 use serde::{Deserialize, Serialize};
-use std::ops::{Index, IndexMut};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Block {
@@ -22,20 +21,6 @@ impl Block {
     }
 }
 
-impl Index<usize> for Block {
-    type Output = Pipeline;
-
-    fn index(&self, index: usize) -> &Self::Output {
-        &self.pipelines[index]
-    }
-}
-
-impl IndexMut<usize> for Block {
-    fn index_mut(&mut self, index: usize) -> &mut Self::Output {
-        &mut self.pipelines[index]
-    }
-}
-
 impl Default for Block {
     fn default() -> Self {
         Self::new()

crates/nu-protocol/src/ast/pipeline.rs

@@ -1,6 +1,5 @@
 use crate::{ast::Expression, engine::StateWorkingSet, Span};
 use serde::{Deserialize, Serialize};
-use std::ops::{Index, IndexMut};
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq)]
 pub enum Redirection {
@@ -159,17 +158,3 @@ impl Pipeline {
         self.elements.is_empty()
     }
 }
-
-impl Index<usize> for Pipeline {
-    type Output = PipelineElement;
-
-    fn index(&self, index: usize) -> &Self::Output {
-        &self.elements[index]
-    }
-}
-
-impl IndexMut<usize> for Pipeline {
-    fn index_mut(&mut self, index: usize) -> &mut Self::Output {
-        &mut self.elements[index]
-    }
-}