locks: Remove the last reference to EXPORT_OP_ASYNC_LOCK.

Commit b875bd5b ("exportfs: Remove EXPORT_OP_ASYNC_LOCK") removed
all references to EXPORT_OP_ASYNC_LOCK, but one lasted in the
comments for fs/locks.c. Remove it.

Signed-off-by: Thiago Becker <tbecker@redhat.com>
Link: https://lore.kernel.org/20250724203516.153616-1-tbecker@redhat.com


Signed-off-by: Christian Brauner <brauner@kernel.org>