Remove make_bitboard()

Remove make_bitboard()

In current master, the function make_bitboard() does nothing apart from
helping initialize the SquareBB[] array. This seems like an unnecessary
abstraction layer.

The advantage of make_bitboard() is we can define a bitboard, in a simple
and general way, not only from a single square but also from a list of
squares. It is more elegant, faster and  readable than combining multiple
SquareBB explicitly, but the last complex use case in evaluation was
simplified away a few months ago.

If make_bitboard() becomes useful again to define complicated bitboards,
it will be easy enough to reintroduce it using this pull request as
an implementation reference.

No functional change.