VACMS-5888: Hide existing VC services from option list to prevent duplicates.

[ethanteague]

Description

Closes #5888

Testing done

Visual

QA steps

• As an administrator, go to node/3756/edit.
• Add a service, and visually verify only options that are not selected in the table above appear.
• Remove items from the table, and verify that they appear in the select.

Definition of Done

• Documentation has been updated, if applicable.
• Tests have been added if necessary.
• Automated tests have passed.
• Code Quality Tests have passed.
• Acceptance Criteria in related issue are met.
• Manual Code Review Approved.

Select Team for PR review

• Core Application Team
• Product Support Team

Is this PR blocked by another PR?

• DO NOT MERGE

Does this PR need review from a Product Owner

• Needs PO review

CMS user-facing annoucement

Is an announcement needed to let editors know of this change?

• Yes, and it's written in issue ____ and queued for publication.
  • Merge and ping @ rachel-kauff so she's ready to publish after deployment
• Yes, but it hasn't yet been written
  • Don't merge yet -- ping @ rachel-kauff to prompt her to write and queue content
• No announcement is needed for this code change.
  • Merge & carry on unburdened by announcements

[kevwalsh]

QA Pass, over to @swirtSJW for CR

[swirtSJW]

I'm not really sure what this comment means.

[swirtSJW]