Remove the vue_group_select feature flag

What does this MR do and why?

This removes the vue_group_select feature flag, which causes the custom templates group selectors to default to the newer implementation, based off GlCollapsibleListbox.

Please refer to the MR that introduced the feature flag for screenshots and instructions on how to verify these changes: !98597 (merged). Note that some additional improvements were shipped in !104308 (merged).

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.

Related to #374010 (closed)