Remove stubbed `vue_group_members_list` feature flag from specs

What does this MR do?

Part of &4233 (closed) and #241194 (closed)

Over the last two milestones I have been working on converting the group members view from HAML to Vue. Because this spanned across multiple milestones I stubbed the feature flag while developing. Now the development is done and I will be enabling the vue_group_members_list feature flag in the next couple of weeks. This MR removes the vue_group_members_list stubbed feature flag and updates selectors in the feature specs. I also took this opportunity to clean up a few feature specs and remove some data-qa-selector selectors as those should only be used in QA specs.

This updates the following specs:

• spec/features/admin/admin_groups_spec.rb
• spec/features/groups/members/filter_members_spec.rb
• spec/features/groups/members/leave_group_spec.rb
• spec/features/groups/members/list_members_spec.rb
• spec/features/groups/members/manage_groups_spec.rb
• spec/features/groups/members/manage_members_spec.rb
• spec/features/groups/members/master_adds_member_with_expiration_date_spec.rb
• spec/features/groups/members/master_manages_access_requests_spec.rb
• spec/features/groups/members/search_members_spec.rb
• spec/features/groups/members/sort_members_spec.rb

These specs will be updated in a subsequent MR:

• ee/spec/features/groups/audit_events_spec.rb
• ee/spec/features/groups/members/list_members_spec.rb
• ee/spec/features/groups/members/override_ldap_memberships_spec.rb

Screenshots (strongly suggested)

Specs only, no visual changes

GIF for context

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
  • Not needed, behind a feature flag
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] 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