Remove unused component

What does this MR do and why?

Remove unused file templates dropdown component

Steps done to verify that the component is unused.

1. No imports/usages -- excluding spec file.

• GitLab code search (none found for "file_templates/dropdown")
• RubyMine's Find in Files (none found "file_templates/dropdown")
• RubyMine's Find usages (none found)

2. No relative imports

• RubyMine's Find in Files (none found for "../dropdown")
• RubyMine's Find in FIles (found one but points to a different component for "./dropdown")

3. Git history -- found MR that replaced its usage.

• Replaced usages with GlDropdown on 76b20c02
• It was strictly used on bar.vue when it was first added gitlab-foss!21245 (diffs)

4. Check git history, see if it might have been re-used anywhere else. Result: Nothing seems to stand out, most were lints and import adjustments.
5. This is part of the old Web IDE which we don't use anymore (needs to verify with the IDE team)
6. All of our first set of pipeline test passes.

Screenshots or screen recordings

No visual change

How to set up and validate locally

N/A

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #418071 (closed)