Fix spec input_copy_toggle_visibility_spec.js

What does this MR do and why?

This is part of the Vue 3 migration > &5102

• Remove Vue instance property assignment

Screenshots or screen recordings

n/a

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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 #413336 (closed)