Reformat promotion generation

Use the same template of other pawns moves generation,
make the code more uniform, simplify generate_promotions
that has now been renamed.

No functional change (verified also with perft).
Remove some comments in timeman.cpp

Commenst are obsolete now, an updated description
would be quite obscure, so better let the code
to talk and remove them all together.

No functional change.
Merge branch 'master' of https://github.com/official-stockfish/Stockfish