MR Memory improvements: Properly cleanup diff_line_note_form.vue

Summary

@iamphill writes in #217950 (comment 362850834)

https://gitlab.com/gitlab-org/gitlab/-/blob/9a2eb9a4a9fd12c8c3cb07f701523c32b7c01fcf/app/assets/javascripts/diffs/components/diff_line_note_form.vue#L101 we init the autosave but never destroy it 🤷

Also whilst here - I noticed https://gitlab.com/gitlab-org/gitlab/-/blob/9a2eb9a4a9fd12c8c3cb07f701523c32b7c01fcf/app/assets/javascripts/diffs/components/diff_line_note_form.vue#L66-68 which seems a bit of a pointless re-assign, not a massive saving, but something to consider change.

Improvements

Risks

Involved components

• /diffs/components/diff_line_note_form.vue

Optional: Intended side effects

After leaving multiple comments, we should see a smaller memory increase than we see before this issue gets solved. (some profiling will be required)