Remove unused roadmapShellEl from milestones list section

What does this MR do and why?

Removes the unused roadmapShellEl data field from both roadmap list sections.

• ee/app/assets/javascripts/roadmap/components/epics_list_section.vue
  • roadmapShellEl in data()
  • its assignment from this.$root.$el.querySelector('.js-roadmap-shell') in initMounted()
• ee/app/assets/javascripts/roadmap/components/milestones_list_section.vue
  • roadmapShellEl in data()
  • its assignment from this.$root.$el.firstChild in initMounted()

Nothing reads the field in either component or in their specs.

Why it went out of use

• 2019-11, !19875 (merged): buffered rendering for the roadmap. roadmapShellEl held the roadmap shell element and was passed as :scrollelement to vue-virtual-scroll-list in epics_list_section.vue, behind the roadmap_buffered_rendering flag.
• 2020-01, !22748 (merged): milestones added to the roadmap. milestones_list_section.vue was modelled on the epics section and copied the field and the assignment, but it never rendered a virtual list. The field was unused in this file from day one.
• 2020-08, !40243 (merged): the epics section changed the lookup from firstChild to querySelector('.js-roadmap-shell') because firstChild did not return the shell. The milestones copy was not updated.
• 2021-03, !57486 (merged): the roadmap_buffered_rendering flag and the <virtual-list> element were removed because buffered rendering had been broken for a while. That removed the only reader of roadmapShellEl. The field and its assignment stayed in both files.

The firstChild lookup was the reason this came up: under Vue 3 it can return a text or comment node instead of an element. Since the field is unused, removing it is the fix.

How to set up and validate locally

yarn jest ee/spec/frontend/roadmap/components/epics_list_section_spec.js ee/spec/frontend/roadmap/components/milestones_list_section_spec.js

The roadmap page has no visible change.

References

• Found while reviewing !251472 (comment 3853520260) (reviewer listed the firstChild line as a second occurrence)
• Related fix for the other occurrence: !256325 (merged)
• Introduced the field: !19875 (merged)
• Copied it to the milestones section: !22748 (merged)
• Changed the epics lookup to querySelector: !40243 (merged)
• Removed its only reader: !57486 (merged)