Ensure VSA form submits only required fields

What does this MR do?

In !64273 (merged) we introduced some changes to store group labels in the vuex store for re-use in the form, this resulted in some additional fields getting added to the edit form fields, the additional fields were appearing in the payload when editing a value stream that has label based stages, causing the PUT request to fail.

Fixes #340073 (closed)

Screenshots or Screencasts (strongly suggested)

Before	After
Screen_Recording_2021-09-06_at_8.22.21_pm	Screen_Recording_2021-09-06_at_8.10.33_pm

How to setup and validate locally (strongly suggested)

Prerequisites:

• Ensure you have an EE license, or stub the cycle_analytics_for_groups license feature to true
• Make sure you have a group created
• Make sure you create some labels for the group

Steps to reproduce

1. Navigate to value stream analytics for the group, ie for a project named clean-vsa-project in group new-vsa-group it would be http:///group/:group_id/-/analytics/value_stream_analytics (or find it in the left menu)
2. Click the value stream selector dropdown next to the page title (if its a new group, default will be the only value stream available)
3. Click Create new Value Stream
4. Select Create from no template
5. Fill all the fields for the custom value stream
   • For the Start event , make sure to select Issue label was added, then select a group label
6. Click Create value stream to save this new value stream
7. Reload the page, then select your new value stream and click the Edit button
8. Edit the End event and click Save value stream

Your value stream should be updated with the latest changes

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related 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.
• This change is backwards compatible across updates, or this does not apply.

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.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #340073 (closed)