[web] Get rid of "square" under messageTooltipMenu

Summary:
Before: https://blob.sh/atul/6b5c.png
After: https://blob.sh/atul/2041.png

Test Plan: NA

Reviewers: varun, benschac, def-au1t, ashoat

Reviewed By: benschac, ashoat

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3301