Make it a default to remove poison from immune units.

This impacts wesnoth.transform_unit, [effect]apply_to=type, and [effect]apply_to=variation.
If desired, the poisoned state can still be explicitly set for immune units; this change
only affects default behavior, making it correspond better to the core rules.


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56019 75b3775d-3801-0410-9e12-e2e43121521d