Remove unusual unicode marks in Differential action dropdown

Summary:
See https://twitter.com/HayleyCAnderson/status/988873585363009536.

Currently, the action dropdown in Differential shows a heavy "X" after "Request Changes" and a heavy checkmark after "Accept Revision".

Although I'm not convinced that the messaging around "Request Changes" is too strong, I do think these marks are out of place in modern Differential. They came from a simpler time when this dropdown had fewer actions, but feel a little weird and inconsistent to me in the modern UI.

Let's try getting rid of them and see how it goes?

Test Plan:

• Viewed these actions in the dropdown, no longer saw the mark icons.
• Grepped for these unicode sequences without getting any other hits.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19405