sched: Remove unneeded function type cast in do_balance_callbacks()

It's a trivial and non-functional change, the removed explicit function
type cast does not convert the function type of struct balance_callback
member (*func), and the cast can be removed.

Signed-off-by: Vladimir Zapolskiy <vz@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260819080159.105433-1-vz@kernel.org