Remove /usr/bin symlinks during package removal

Use the example of https://github.com/opscode/omnibus-chef-server/blob/ce461819eef3af862c620c449a81b7a13c92fc62/package-scripts/chef-server-core/postrm .

One has to be careful about the RPMs, see http://tickets.opscode.com/browse/CHEF-3022 .