pageranges.py: remove cond::isblogpage as unneeded

When I created cond::isblogpage I didn't realize I already had a
conditional renderer for this exact condition (using almost the same
code), cond::pageinblog. This makes cond::isblogpage surplus, so I've
removed it. I've also tightened up the code of cond::pageinblog.

Oh the embarrassment.
(I think this is one of the hazards of working infrequently on a
sprawling mess.)