[B+C] Adds BUKKIT-4014. Add API for removing scores per player per objective.

Justification:
Scoreboard scores should be removable by any combination of player & objective.

Breakdown:
Add `isSet()`, `reset()` to Score.

Testing:
It compiles, does not work because it doesn't send update packets to the client.

Related PRs:
https://github.com/Bukkit/Bukkit/pull/848

Ticket:
https://bukkit.atlassian.net/browse/BUKKIT-4014