Remove the NU_DISABLE_IR option (#14293)

# Description

Removes the `NU_DISABLE_IR` option and some code related to evaluating
blocks with the AST
evaluator.

Does not entirely remove the AST evaluator yet. We still have some
dependencies on expression
evaluation in a few minor places which will take a little bit of effort
to fix.

Also changes `debug profile` to always include instructions, because the
output is a little
confusing otherwise, and removes the different options for
instructions/exprs.

# User-Facing Changes

- `NU_DISABLE_IR` no longer has any effect, and is removed. There is no
way to use the AST
  evaluator.
- `debug profile` no longer has `--exprs`, `--instructions` options.
- `debug profile` lists `pc` and `instruction` columns by default now.

# Tests + Formatting

Eval tests fixed to only use IR.

# After Submitting

- [ ] release notes
- [ ] finish removing AST evaluator, come up with solutions for the
expression evaluation.

benches/benchmarks.rs

@@ -46,9 +46,6 @@ fn setup_stack_and_engine_from_command(command: &str) -> (Stack, EngineState) {
 
     let mut stack = Stack::new();
 
-    // Support running benchmarks without IR mode
-    stack.use_ir = std::env::var_os("NU_DISABLE_IR").is_none();
-
     evaluate_commands(
         &commands,
         &mut engine,