[web] Remove MessageTimestampTooltip from ChatMessageList

Summary:
Remove the one usage of MessageTimestampTooltip in web.

Next diffs:

1. Remove MessageTimestampTooltip component and associated styles.
2. Display timestamp as part of MessageActionButtons component.

Test Plan:

1. Made sure there were no other usages of the MessageTimestampTooltip component.
2. Made sure that the timestamp tooltip was no longer displayed when hovering over a message.

Reviewers: def-au1t, palys-swm, benschac, ashoat

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3567