Apollo boards - Remove usage of VueX variables

What does this MR do and why?

Apollo boards - Remove usage of VueX variables

This MR cleans up some unused VueX variables and fixes using loading variables for Apollo boards.

Behind :apollo_boards feature flag

Screenshots or screen recordings

The only visual change is the loading state for Swimlanes.

How to set up and validate locally

1. Turn on :apollo_boards feature flag
2. Visit a board
3. In the top right corner, select Group by > Epic
4. Check epics loading state

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.