RCA AI: revert gold saving default back to 'off'

This was discussed both on the wesnoth-dev mailing list and on the
forums and the overall consensus was that it should be off, although
some scenarios will benefit from it.  It should be enabled specifically
in such scenarios as they are identified.
Reference:
https://mail.gna.org/public/wesnoth-dev/2014-01/msg00001.html
and links therein