Push some processing from unit.cpp to unit_type.cpp. This accomplishes two things:
1) A list of unit_type attributes gets moved to a more fitting file (unit_type.cpp).
2) Trimming down a unit_type config for units occurs once per (used) unit type, rather
   than once per unit.


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56312 75b3775d-3801-0410-9e12-e2e43121521d