More efficient version of find_routes().

This removes some overhead that accommodated edge-weights (which is
not needed since Wesnoth maps bear vertex weights). Testing indicates
a reduction in execution times on the order of 1%-4%. Not much, but
this function is called fairly often.

This is an updated version of (my) patch #3122.


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56379 75b3775d-3801-0410-9e12-e2e43121521d