[web] remove old Label ui component from add members modal

Summary:
PLEASE NOTE THAT THIS DIFF AND SUBSEQUENT DIFFS IN THIS STACK WILL NOT BE LANDED UNTIL MORE OF THE REDESIGN IS READY SINCE THIS WILL CAUSE REGRESSIONS IN PROD

Now that AddUsersList is completely feature agnostic, we should use this component in the AddMembersModal. As a first step to that we should remove the old "labels" ux

Depends on D10940

Test Plan: flow

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10941