futex: Remove put_futex_key()

Since 4b39f99c ("futex: Remove {get,drop}_futex_key_refs()"),
put_futex_key() is empty.

Remove all references for this function and the then redundant labels.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200702202843.520764-2-andrealmeid@collabora.com