Revert: "languages: Improve function parameter highlighting for Bash" (#52245)

## Context

<!-- What does this PR do, and why? How is it expected to impact users?
     Not just what changed, but what motivated it and why this approach.

Link to Linear issue (e.g., ENG-123) or GitHub issue (e.g., Closes #456)
     if one exists — helps with traceability. -->

Closes https://github.com/zed-industries/zed/issues/52242

This PR reverts a change that highlights shell script arguments as
variables rather than strings. Because of how shell scripts work, all
parameters are strings by default and are regularly represented as such.
The original PR (https://github.com/zed-industries/zed/pull/48067) aimed
to improve highlighting by capturing all arguments as
`variable.parameter`s, but this doesn't align with the idiomatic usage
of shell scripts.

| Current Behavior | Expected Behavior |
|---|---|
| <img width="242" height="84" alt="Image"
src="https://github.com/user-attachments/assets/50c87198-e4be-4627-a742-88060b2b9d14"
/> | <img width="227" height="83" alt="Image"
src="https://github.com/user-attachments/assets/f08b1c62-9c7a-460e-b306-4ac6b9e5a897"
/> |
| Text arguments are highlighted as variable | Text arguments are
highlighted as "string" |

## Self-Review Checklist

<!-- Check before requesting review: -->
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [ ] Performance impact has been considered and is acceptable

Release Notes:

- N/A or Added/Fixed/Improved ...

crates/grammars/src/bash/highlights.scm

@@ -49,13 +49,6 @@
 (command_name
   (word) @function)
 
-(command
-  argument: [
-    (word) @variable.parameter
-    (_
-      (word) @variable.parameter)
-  ])
-
 [
   (file_descriptor)
   (number)