/team join and /team leave tab-complete all entries from scoreboard including fake players

Relates to MC-124056

The bug

/team join and /team leave tab-complete all entries currently stored in the scoreboard including fake players whose names start with #.

/team leave should only tab-complete the names of players / entities which are in the team (currently not the case unless the scoreboard tracks them as well).
(?) Maybe /team join should tab-complete the names of currently online players.

How to reproduce

1. Create a scoreboard objective and store a value for a fake player

   /scoreboard objectives add test dummy
   /scoreboard players set #fakePlayer test 1
   

2. Type the following

   /team leave 
   

   → It suggests #fakePlayer