Partially clean up _determine_armour_ego

The only change in behaviour should be sub-percent differences
in armour ego weights for body armours, & the removal of an
ancient and truly mysterious Archmagi ego special case. (It was
guaranteed to never appear on ego re-rolls, for reasons that
crawl developers appear to have been baffled by for at least the
last seven years.)

If Archmagi becomes too common after this change, there are
simpler ways to adjust the frequency...