[web] Remove MessageTimestampTooltip component and associated styles
Summary:
Remove the MessageTimestampTooltip which we will no longer need once we incorporate message timestamp into the MessageActionButtons component.
Next diff:
- Display timestamp as part of MessageActionButtons component.
Depends on D3567
Test Plan: flow? all usages of component were removed in the previous diff
Reviewers: ashoat, def-au1t, palys-swm, benschac
Reviewed By: ashoat
Subscribers: Adrian, karol-bisztyga, yayabosh
Differential Revision: https://phabricator.ashoat.com/D3570