For simplification, we remove some unused features. Those features can
still be reimplemented externally if needed.

For example, the "rbm rpm" command can be replaced with
"rbm build --step rpm" and a definition of an rpm step in rbm.conf.

We remove the following options:
 - version_command
 - rpm
 - rpmbuild
 - srpm
 - rpmspec
 - rpm_rel
 - debian_create
 - deb
 - deb_src
 - use_pbuilder
 - debian_revision

And the following commands:
 - rpmspec
 - rpm
 - srpm
 - deb-src
 - deb
 - pkg
 - publish