Remove the old release making instructions and scripts.

Using git-based scripts seems to work well and the old scripts are not
maintained any more and having several ways of making releases is just
confusing, so leave only one set of scripts, the one actually used since
2.9.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76710 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775