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-parser/tests/test_parser_unicode_escapes.rs

@@ -19,9 +19,9 @@ pub fn do_test(test: &[u8], expected: &str, error_contains: Option<&str>) {
     match working_set.parse_errors.first() {
         None => {
             assert_eq!(block.len(), 1);
-            let expressions = &block[0];
+            let expressions = &block.pipelines[0];
             assert_eq!(expressions.len(), 1);
-            if let PipelineElement::Expression(_, expr) = &expressions[0] {
+            if let PipelineElement::Expression(_, expr) = &expressions.elements[0] {
                 assert_eq!(expr.expr, Expr::String(expected.to_string()))
             } else {
                 panic!("Not an expression")