Remove wrapper.destroy mentions from frontend docs

What does this MR do and why?

As per our docs https://docs.gitlab.com/ee/development/testing_guide/frontend_testing.html#ensuring-that-tests-are-isolated:

With enableAutoDestroy, it is no longer necessary to manually call wrapper.destroy(). However, some mocks, spies, and fixtures do need to be torn down, and we can leverage the afterEach hook.

Screenshots or screen recordings

N.A.

How to set up and validate locally

Open the docs in the review app.

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 #395023 (closed)