[Feature flag] Cleanup invite_members_group_modal - remove import

What does this MR do and why?

clean up a few remaining items from invite member feature flag rollout

• date/user selector logic only used on old non modal invite form
• import/apply import items only used before import project members modal existed(was rolled out as part of this effort as well)
• Update docs

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 #352526 (closed)