[web] deprecate stringForReactionList

Summary:
In D8708 we got rid of the last usage of stringForReactionList. Now that this function is no longer being used we can get rid of this funciton and the unit tests associated with this function

Depends on D8708

Test Plan: flow and did a global search of stringForReactionList to confirm that this function was not being used anywhere else in the codebase

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8709