perf bench futex: Remove support for IMMUTABLE

It has been decided to remove the support IMMUTABLE futex.
perf bench was one of the eary users for testing purposes. Now that the
API is removed before it could be used in an official release, remove
the bits from perf, too.

Remove Remove support for IMMUTABLE futex.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250710110011.384614-7-bigeasy@linutronix.de