Simplify popcnt

Simplify popcnt

Also a speedup(about 1%) on 64-bit w/o hardware popcnt

Retire Max15 and Full template parameters
(Contributed by Marco Costalba)

Now that we have just SW and HW versions, use
template default parameter to get rid of explicit
template parameters.

Retire bitcount.h and move the only defined
function to bitboard.h

No functional change

Resolves #620