locking/refcounts: Remove stale comment from the ARCH_HAS_REFCOUNT Kconfig entry

locking/refcounts: Remove stale comment from the ARCH_HAS_REFCOUNT Kconfig entry

ARCH_HAS_REFCOUNT is no longer marked as broken ('if BROKEN'), so remove
the stale comment regarding it being broken.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20171229195303.17781-1-ebiggers3@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>