Micro-optimze extension()

Explicitly write the conditions for pawn to 7th
and passed pawn instead of wrapping in redundant
helpers.

Also retire the now unused move_is_pawn_push_to_7th()
and the never used move_was_passed_pawn_push() and
move_is_deep_pawn_push()

Function extension() is so time critical that this
simple patch speeds up the pgo compile of 0.5% and
it is also more clear what actually happens there.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>