[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:
Read the diff, it's 6 lines of code.

Testing:
It compiles (will actually test, haven't yet).

Related PRs:
<will fill in about 10 minutes>

Ticket:
https://bukkit.atlassian.net/browse/BUKKIT-4014