Remove item_mass

Was being used for: fedhas fungal blooms, jelly healing when
eating items, projectile noise, awkward throwing range, and
autotraining of Armour, Dodging, and Stealth. (All based on
body armour mass.)

The former is unchanged (switched to using source monster mass
rather than item mass). Jelly healing now gives a flat +3 hp per
item eaten. Projectile noise uses (projectile damage/3) instead of
(sqrt(item_mass) / 3 + 0.5). Awkward throwing range is now always
to edge of LOS (it essentially was anyway, for anything lighter
than plate & any character stronger than a DEWz). Skill training
uses a linear approximation of the old values.