selftests/futex: Remove support for IMMUTABLE

Testing for the IMMUTABLE part of the futex interface is not needed
after the removal of the interface.

Remove support for IMMUTABLE from the sefltest.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250710110011.384614-6-bigeasy@linutronix.de