mon/Paxos: don't spam log with is_readable at dout level 1

Backport: firefly, dumpling Reported-by: Aanchal Agrawal
<Aanchal.Agrawal@sandisk.com> Signed-off-by: Sage Weil <sage@redhat.com>
Why should this even be at level 2?
I assume this output was added/bumped up for some debugging purpose, but I don't see knowing that it's readable as being that much more useful than knowing the rest of what's going on with the system...