Remove @:generic from Promise.

[player-03]

This was first added in 3dad6d8, without explanation. I think it's time to remove it.

Considerations:

• In theory, @:generic may make the code faster. However, this isn't performance-critical code, and any improvements are likely to be vanishingly small.
• @:generic increases the size of the output.
• @:generic prevents casting to Promise<Dynamic>, which forced a slightly more verbose implementation of FutureWork. Technically this enforces type safety, but usually we don't worry about that sort of thing. If a user's going to use Dynamic, they accept responsibility for the risks.
• If we remove the tag, we'll be able to add static utility functions.