Revert #7779 (enables back subcommand completions) (#8102)

# Description

Reverts the PR #7779 which breaks subcommand completions. The issues
#7648 and #7754 thus still need fixing.

This reverts commit 8acced5.

# User-Facing Changes

Enables subcommand completions.

Unfortunately, also brings back the completion panic if alias is shorter
than the command name.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-cli/tests/completions.rs

@@ -819,8 +819,9 @@ fn extern_complete_flags(mut extern_completer: NuCompleter) {
     match_suggestions(expected, suggestions);
 }
 
+#[ignore = "was reverted, still needs fixing"]
 #[rstest]
-fn alias_offset_bug_7748() {
+fn alias_offset_bug_7648() {
     let (dir, _, mut engine, mut stack) = new_engine();
 
     // Create an alias
@@ -829,15 +830,15 @@ fn alias_offset_bug_7748() {
 
     let mut completer = NuCompleter::new(std::sync::Arc::new(engine), stack);
 
-    // Issue #7748
+    // Issue #7648
     // Nushell crashes when an alias name is shorter than the alias command
     // and the alias command is a external command
     // This happens because of offset is not correct.
     // This crashes before PR #7779
     let _suggestions = completer.complete("e", 1);
-    //println!(" --------- suggestions: {:?}", suggestions);
 }
 
+#[ignore = "was reverted, still needs fixing"]
 #[rstest]
 fn alias_offset_bug_7754() {
     let (dir, _, mut engine, mut stack) = new_engine();
@@ -853,8 +854,6 @@ fn alias_offset_bug_7754() {
     // and the alias command contains pipes.
     // This crashes before PR #7756
     let _suggestions = completer.complete("ll -a | c", 9);
-
-    //println!(" --------- suggestions: {:?}", suggestions);
 }
 
 #[test]