Have unit::advance_to() no longer change the unit's current hit points.

This was overridden more often than not anyway, and the code is overall simpler
without hit points being changed by this function.


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56016 75b3775d-3801-0410-9e12-e2e43121521d