Fix free-variable finder

Fix free-variable finder

After a long hunt I discovered that the reason that GHC.Enum.eftIntFB
was being marked as a loop-breaker was the bizare behaviour of exprFreeVars,
which returned not only the free variables of an expression but also the
free variables of RULES attached to variables occuring in the expression!

This was clearly deliberate (the comment was CoreFVs rev 1.1 in 1999) but
I've removed it; I've left the comment with further notes in case there
turns out to be a Deep Reason.