Removes the VSA customStages vuex module

What does this MR do?

Further follow-up to #323984 (closed)

This MR removes the customStages vuex module, we no longer require this module as the CRUD operations on value stream stages now all occur from the value_stream_form.vue.

In addition, we need to move the formEvents state key to the vuex root to continue to have access to this data. Related tests are updated to reflect this change.

This MR:

• Removes the vuex module
• Updates tests
• Moves the formEvents from the customStages module to the vuex top level

Screenshots (strongly suggested)

No visual changes in this MR

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?) - no visual changes
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #323984 (closed)