Retire one implementation of pop_lsb()

We have two implementations that are equivalent,
so retire one.

Plus usual tidy up of comments and code reshuffle.

No functional change.

src/main.cpp

@@ -35,7 +35,7 @@ int main(int argc, char* argv[]) {
   UCI::init(Options);
   Bitboards::init();
   Position::init();
-  Bitbases::init_kpk();
+  Bitbases::init();
   Search::init();
   Eval::init();
   Pawns::init();