Relocate unregistration of player on respawn. Fixes BUKKIT-4561.

### The Issue:
The temporary removal from the PlayerList of a respawning EntityPlayer
currently occurs before the PlayerRespawnEvent is fired, meaning that
the player cannot be retrieved by name during the event.  This commit
relocates said removal to occur after the event has fired, resolving the
issue without altering visible vanilla behavior (untracking and other entity
removal operations are left intact).
### The Justification:
As a result of the proposed relocation of player unregistration, retr