Removed IgWall unit type flag. Rulesets use "present = FALSE"
requirements for Defend_Bonus effects to achieve what IgWall used
to provide.

See patch #4799


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25273 a0f10bec-cc02-0410-94fc-a9cfff90b4cd