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.

crates/nu-cli/src/repl.rs

@@ -306,9 +306,6 @@ fn loop_iteration(ctx: LoopContext) -> (bool, Stack, Reedline) {
     if let Err(err) = engine_state.merge_env(&mut stack) {
         report_shell_error(engine_state, &err);
     }
-    // Check whether $env.NU_DISABLE_IR is set, so that the user can change it in the REPL
-    // Temporary while IR eval is optional
-    stack.use_ir = !stack.has_env_var(engine_state, "NU_DISABLE_IR");
     perf!("merge env", start_time, use_color);
 
     start_time = std::time::Instant::now();