Used a consistent tense for system messages (#8079)

app/team.go

@@ -681,7 +681,7 @@ func (a *App) postRemoveFromTeamMessage(user *model.User, channel *model.Channel
 		Type:      model.POST_REMOVE_FROM_TEAM,
 		UserId:    user.Id,
 		Props: model.StringInterface{
-			"removedUsername": user.Username,
+			"username": user.Username,
 		},
 	}