action: refactor ATOMIC_READ printing

action: refactor ATOMIC_READ printing

For ATOMIC_READ, we don't need a special case to print VALUE_NONE,
because any action where the value isn't explicitly set (e.g., all
ATOMIC_READ actions) default to VALUE_NONE. And if this 'default'
changes in the future, we would want to print it.

(Right now it's confusing, for instance, when we have a failed promise
execution, since the model-checker trace gives no indication of what the
promised value actually *was*. This should be fixed sometime.)