futex/requeue: Remove unnecessary ‘NULL’ initialization from futex_proxy_trylock_atomic()

'top_waiter' is assigned unconditionally before first use,
so it does not need an initialization.

[ mingo: Created legible changelog. ]

Signed-off-by: Li zeming <zeming@nfschina.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230725195047.3106-1-zeming@nfschina.com