Drop unused plain javascript file

What does this MR do?

As part of #21320 we are removing inline javascript files. Here I'm removing app/views/admin/runners/update.js.haml which is unused.

See rationale here #21320 (comment 404668635)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done