icount: Remove confusing statement about inhibited trigger.

"Inhibited" is not well-defined, and this language is no longer
necessary. It was added when the original sentence said:
> When \FcsrIcountCount is greater than 1, every instruction completed or
> trap taken from a mode where the trigger is enabled decrements
> \FcsrIcountCount by 1.

Now there is no more reference to a mode, so the clarification isn't
needed either.

Addresses #813